fix SEGV touching uninitialized memory

This function can be called from Init_VM().
No assumption can be made about object internals.

(lldb) run
Process 15734 launched: './miniruby' (x86_64)
Process 15734 stopped
* thread #1: tid = 0x1441d4, 0x00000001000bdfcb miniruby`rb_raw_iseq_info(buff="0x0000000100f61f48 [0    ] T_IMEMO iseq", buff_size=256, iseq=0x0000000100f61f48) + 27 at gc.c:9273, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x50)
    frame #0: 0x00000001000bdfcb miniruby`rb_raw_iseq_info(buff="0x0000000100f61f48 [0    ] T_IMEMO iseq", buff_size=256, iseq=0x0000000100f61f48) + 27 at gc.c:9273
   9270 static void
   9271 rb_raw_iseq_info(char *buff, const int buff_size, const rb_iseq_t *iseq)
   9272 {
-> 9273     if (iseq->body->location.label) {
   9274         VALUE path = rb_iseq_path(iseq);
   9275         snprintf(buff, buff_size, "%s %s@%s:%d", buff,
   9276                  RSTRING_PTR(iseq->body->location.label),
(lldb) p *iseq
(rb_iseq_t) $0 = {
  flags = 28698
  reserved1 = 0
  body = 0x0000000000000000
  aux = {
    compile_data = 0x0000000000000000
    loader = (obj = 0, index = 0)
    trace_events = 0
  }
}
(lldb) bt
* thread #1: tid = 0x1441d4, 0x00000001000bdfcb miniruby`rb_raw_iseq_info(buff="0x0000000100f61f48 [0    ] T_IMEMO iseq", buff_size=256, iseq=0x0000000100f61f48) + 27 at gc.c:9273, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x50)
  * frame #0: 0x00000001000bdfcb miniruby`rb_raw_iseq_info(buff="0x0000000100f61f48 [0    ] T_IMEMO iseq", buff_size=256, iseq=0x0000000100f61f48) + 27 at gc.c:9273
    frame #1: 0x00000001000bde72 miniruby`rb_raw_obj_info(buff="0x0000000100f61f48 [0    ] T_IMEMO iseq", buff_size=256, obj=4311097160) + 2786 at gc.c:9396
    frame ruby#2: 0x00000001000b7c5f miniruby`obj_info(obj=4311097160) + 95 at gc.c:9428
    frame ruby#3: 0x00000001000c16a8 miniruby`newobj_init(klass=0, flags=28698, v1=0, v2=0, v3=0, wb_protected=1, objspace=0x00000001007ee280, obj=4311097160) + 424 at gc.c:1887
    frame ruby#4: 0x00000001000b44c9 miniruby`newobj_of(klass=0, flags=28698, v1=0, v2=0, v3=0, wb_protected=1) + 217 at gc.c:1970
    frame ruby#5: 0x00000001000b464b miniruby`rb_imemo_new(type=imemo_iseq, v1=0, v2=0, v3=0, v0=0) + 75 at gc.c:2017
    frame ruby#6: 0x00000001000fd914 miniruby`iseq_imemo_alloc + 36 at iseq.h:156
    frame ruby#7: 0x00000001000f6e1d miniruby`iseq_alloc + 13 at iseq.c:211
    frame ruby#8: 0x00000001000f6bf8 miniruby`rb_iseq_new_with_opt(node=0x0000000000000000, name=4311097200, path=4311097200, realpath=8, first_lineno=1, parent=0x0000000000000000, type=ISEQ_TYPE_TOP, option=0x0000000100335c30) + 56 at iseq.c:519
    frame ruby#9: 0x00000001000f6bb6 miniruby`rb_iseq_new(node=0x0000000000000000, name=4311097200, path=4311097200, realpath=8, parent=0x0000000000000000, type=ISEQ_TYPE_TOP) + 86 at iseq.c:480
    frame ruby#10: 0x0000000100284bb0 miniruby`Init_VM + 1040 at vm.c:3022
    frame ruby#11: 0x00000001000d4f7d miniruby`rb_call_inits + 189 at inits.c:55
    frame ruby#12: 0x000000010009fe06 miniruby`ruby_setup + 198 at eval.c:61
    frame ruby#13: 0x000000010009fe5d miniruby`ruby_init + 13 at eval.c:78
    frame ruby#14: 0x00000001000009ed miniruby`main(argc=2, argv=0x00007fff5fbfdbf0) + 93 at main.c:41
    frame ruby#15: 0x00007fff88eda5ad libdyld.dylib`start + 1
(lldb)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61564 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: shyouhei

gc.c

@@ -9270,7 +9270,7 @@ method_type_name(rb_method_type_t type)
 static void
 rb_raw_iseq_info(char *buff, const int buff_size, const rb_iseq_t *iseq)
 {
-    if (iseq->body->location.label) {
+    if (iseq->body && iseq->body->location.label) {
 	VALUE path = rb_iseq_path(iseq);
 	snprintf(buff, buff_size, "%s %s@%s:%d", buff,
 		 RSTRING_PTR(iseq->body->location.label),