check ep during compaction because it can be null

This commit adds a check on the ep just like in the mark function.  The
env can contain null bytes if allocation tracing is enabled.

We're seeing errors during autocompaction like this:

```
(lldb) bt 40
* thread #1, name = 'ruby', stop reason = signal SIGABRT
    frame #0: 0x00007f7d64b6018b libc.so.6`raise + 203
    frame #1: 0x00007f7d64b3f859 libc.so.6`abort + 299
    frame #2: 0x000055af5f2fefc9 ruby`die at error.c:764:5
    frame #3: 0x000055af5f2ff1ac ruby`rb_bug_for_fatal_signal(default_sighandler=0x0000000000000000, sig=11, ctx=0x000055af60bc3340, fmt="") at error.c:804:5
    frame #4: 0x000055af5f4bd08f ruby`sigsegv(sig=11, info=0x000055af60bc3470, ctx=0x000055af60bc3340) at signal.c:960:5
    frame #5: 0x00007f7d64ebe3c0 libpthread.so.0`__restore_rt
    frame #6: 0x000055af5f339b0a ruby`gc_ref_update_imemo(objspace=0x000055af60b2b040, obj=0x00007f7d5b513fd0) at gc.c:9046:13
    frame #7: 0x000055af5f339172 ruby`gc_update_object_references(objspace=0x000055af60b2b040, obj=0x00007f7d5b513fd0) at gc.c:9307:9
    frame #8: 0x000055af5f338e79 ruby`gc_ref_update(vstart=0x00007f7d5b510010, vend=0x00007f7d5b513ff8, stride=40, objspace=0x000055af60b2b040, page=0x000055af62577aa0) at gc.c:9452:21
    frame #9: 0x000055af5f337846 ruby`gc_update_references(objspace=0x000055af60b2b040, heap=0x000055af60b2b068) at gc.c:9481:9
    frame #10: 0x000055af5f336569 ruby`gc_compact_finish(objspace=0x000055af60b2b040, heap=0x000055af60b2b068) at gc.c:4840:5
    frame #11: 0x000055af5f335efb ruby`gc_page_sweep(objspace=0x000055af60b2b040, heap=0x000055af60b2b068, sweep_page=0x000055af63a1eb30) at gc.c:5046:13
    frame #12: 0x000055af5f3355c5 ruby`gc_sweep_step(objspace=0x000055af60b2b040, heap=0x000055af60b2b068) at gc.c:5214:19
    frame #13: 0x000055af5f33daf6 ruby`gc_sweep_rest(objspace=0x000055af60b2b040) at gc.c:5271:2
    frame #14: 0x000055af5f33cacd ruby`gc_sweep(objspace=0x000055af60b2b040) at gc.c:5389:2
    frame #15: 0x000055af5f33c21d ruby`gc_marks_rest(objspace=0x000055af60b2b040) at gc.c:7555:5
    frame #16: 0x000055af5f324d41 ruby`gc_rest(objspace=0x000055af60b2b040) at gc.c:8457:13
    frame #17: 0x000055af5f3297d8 ruby`garbage_collect(objspace=0x000055af60b2b040, reason=45568) at gc.c:8318:9
    frame #18: 0x000055af5f344ece ruby`garbage_collect_with_gvl(objspace=0x000055af60b2b040, reason=45568) at gc.c:8632:9
    frame #19: 0x000055af5f344e61 ruby`objspace_malloc_gc_stress(objspace=0x000055af60b2b040) at gc.c:10592:9
    frame #20: 0x000055af5f32ced1 ruby`objspace_xmalloc0(objspace=0x000055af60b2b040, size=64) at gc.c:10767:5
    frame #21: 0x000055af5f32ce11 ruby`ruby_xmalloc0(size=64) at gc.c:10988:12
    frame #22: 0x000055af5f32cdac ruby`ruby_xmalloc_body(size=64) at gc.c:10997:12
    frame #23: 0x000055af5f329415 ruby`ruby_xmalloc(size=64) at gc.c:12942:12
    frame #24: 0x00007f7d611c4fe5 objspace.so`newobj_i(tpval=0x00007f7d5b553770, data=0x000055af639031a0) at object_tracing.c:101:35
    frame #25: 0x000055af5f5b283f ruby`tp_call_trace(tpval=0x00007f7d5b553770, trace_arg=0x00007fff1016d398) at vm_trace.c:1115:2
    frame #26: 0x000055af5f5b50ec ruby`exec_hooks_body(ec=0x000055af60b2b700, list=0x000055af60b2b920, trace_arg=0x00007fff1016d398) at vm_trace.c:304:3
    frame #27: 0x000055af5f5b0f24 ruby`exec_hooks_unprotected(ec=0x000055af60b2b700, list=0x000055af60b2b920, trace_arg=0x00007fff1016d398) at vm_trace.c:333:5
    frame #28: 0x000055af5f5b0da8 ruby`rb_exec_event_hooks(trace_arg=0x00007fff1016d398, hooks=0x000055af60b2b920, pop_p=0) at vm_trace.c:378:13
    frame #29: 0x000055af5f33f8e2 ruby`rb_exec_event_hook_orig(ec=0x000055af60b2b700, hooks=0x000055af60b2b920, flag=1048576, self=0x00007f7d5b5c08c0, id=0, called_id=0, klass=0x0000000000000000, data=0x00007f7d5b513fd0, pop_p=0) at vm_core.h:1989:5
    frame #30: 0x000055af5f334975 ruby`gc_event_hook_body(ec=0x000055af60b2b700, objspace=0x000055af60b2b040, event=1048576, data=0x00007f7d5b513fd0) at gc.c:2083:5
  * frame #31: 0x000055af5f3342df ruby`newobj_slowpath_wb_protected [inlined] newobj_slowpath(klass=0x00007f7d5b9d19c8, flags=0x000000000000001a, objspace=0x000055af60b2b040, cr=0x000055af60b2b910, wb_protected=1) at gc.c:2284:9
    frame #32: 0x000055af5f33410f ruby`newobj_slowpath_wb_protected(klass=0x00007f7d5b9d19c8, flags=0x000000000000001a, objspace=0x000055af60b2b040, cr=0x000055af60b2b910) at gc.c:2299
    frame #33: 0x000055af5f333de9 ruby`newobj_of0(klass=0x00007f7d5b9d19c8, flags=0x000000000000001a, wb_protected=1, cr=0x000055af60b2b910) at gc.c:2338:11
    frame #34: 0x000055af5f3227ae ruby`newobj_of(klass=0x00007f7d5b9d19c8, flags=0x000000000000001a, v1=0x000055af657d88a0, v2=0x000055af657d8890, v3=0x0000000000000000, wb_protected=1) at gc.c:2348:17
    frame #35: 0x000055af5f322c5b ruby`rb_imemo_new(type=imemo_env, v1=0x000055af657d88a0, v2=0x000055af657d8890, v3=0x0000000000000000, v0=0x00007f7d5b9d19c8) at gc.c:2434:12
    frame #36: 0x000055af5f5a3925 ruby`vm_env_new(env_ep=0x000055af657d88a0, env_body=0x000055af657d8890, env_size=4, iseq=0x00007f7d5b9d19c8) at vm_core.h:1363:33
    frame #37: 0x000055af5f5a3808 ruby`vm_make_env_each(ec=0x000055af60b2b700, cfp=0x00007f7d6482fc90) at vm.c:801:11
    frame #38: 0x000055af5f5a368d ruby`vm_make_env_each(ec=0x000055af60b2b700, cfp=0x00007f7d6482fc20) at vm.c:752:13
    frame #39: 0x000055af5f5a368d ruby`vm_make_env_each(ec=0x000055af60b2b700, cfp=0x00007f7d6482fbb0) at vm.c:752:13
(lldb) f 31
frame #31: 0x000055af5f3342df ruby`newobj_slowpath_wb_protected [inlined] newobj_slowpath(klass=0x00007f7d5b9d19c8, flags=0x000000000000001a, objspace=0x000055af60b2b040, cr=0x000055af60b2b910, wb_protected=1) at gc.c:2284:9
   2281	        }
   2282	        GC_ASSERT(obj != 0);
   2283	        newobj_init(klass, flags, wb_protected, objspace, obj);
-> 2284	        gc_event_hook_prep(objspace, RUBY_INTERNAL_EVENT_NEWOBJ, obj, newobj_fill(obj, 0, 0, 0));
   2285	    }
   2286	    RB_VM_LOCK_LEAVE_CR_LEV(cr, &lev);
   2287
(lldb) p obj
(VALUE) $3 = 0x00007f7d5b513fd0
(lldb) f 6
frame #6: 0x000055af5f339b0a ruby`gc_ref_update_imemo(objspace=0x000055af60b2b040, obj=0x00007f7d5b513fd0) at gc.c:9046:13
   9043	        {
   9044	            rb_env_t *env = (rb_env_t *)obj;
   9045	            TYPED_UPDATE_IF_MOVED(objspace, rb_iseq_t *, env->iseq);
-> 9046	            UPDATE_IF_MOVED(objspace, env->ep[VM_ENV_DATA_INDEX_ENV]);
   9047	            gc_update_values(objspace, (long)env->env_size, (VALUE *)env->env);
   9048	        }
   9049	        break;
(lldb) p obj
(VALUE) $4 = 0x00007f7d5b513fd0
(lldb)
```

gc.c

@@ -9028,9 +9028,12 @@ gc_ref_update_imemo(rb_objspace_t *objspace, VALUE obj)
       case imemo_env:
         {
             rb_env_t *env = (rb_env_t *)obj;
-            TYPED_UPDATE_IF_MOVED(objspace, rb_iseq_t *, env->iseq);
-            UPDATE_IF_MOVED(objspace, env->ep[VM_ENV_DATA_INDEX_ENV]);
-            gc_update_values(objspace, (long)env->env_size, (VALUE *)env->env);
+            if (LIKELY(env->ep)) {
+                // just after newobj() can be NULL here.
+                TYPED_UPDATE_IF_MOVED(objspace, rb_iseq_t *, env->iseq);
+                UPDATE_IF_MOVED(objspace, env->ep[VM_ENV_DATA_INDEX_ENV]);
+                gc_update_values(objspace, (long)env->env_size, (VALUE *)env->env);
+            }
         }
         break;
       case imemo_cref: