Avoid segfault when ptls->safepoint is NULL

There are cases (eg: when __gmpz_init is called from python via PyCall in a
secondary thread) where ptls->safepoint is NULL when maybe_collect is called,
and *ptls->safepoint causes a NULL pointer dereference and a segfault.  This
change fixes that case.

See JuliaLang#2720 for backtrace and steps to reproduce.

src/gc.c

@@ -2744,23 +2744,29 @@ void jl_gc_init(void)
 JL_DLLEXPORT void *jl_gc_counted_malloc(size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    maybe_collect(ptls);
+    if (ptls && ptls->safepoint) {
+        maybe_collect(ptls);
+    }
     gc_num.allocd += sz;
     gc_num.malloc++;
     void *b = malloc(sz);
-    if (b == NULL)
+    // If ptls->safepoint is NULL, then let the caller deal with the failed malloc
+    if (b == NULL && ptls && ptls->safepoint)
         jl_throw(jl_memory_exception);
     return b;
 }
 
 JL_DLLEXPORT void *jl_gc_counted_calloc(size_t nm, size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    maybe_collect(ptls);
+    if (ptls && ptls->safepoint) {
+        maybe_collect(ptls);
+    }
     gc_num.allocd += nm*sz;
     gc_num.malloc++;
     void *b = calloc(nm, sz);
-    if (b == NULL)
+    // If ptls->safepoint is NULL, then let the caller deal with the failed calloc
+    if (b == NULL && ptls && ptls->safepoint)
         jl_throw(jl_memory_exception);
     return b;
 }
@@ -2781,14 +2787,17 @@ JL_DLLEXPORT void jl_gc_counted_free(void *p, size_t sz)
 JL_DLLEXPORT void *jl_gc_counted_realloc_with_old_size(void *p, size_t old, size_t sz)
 {
     jl_ptls_t ptls = jl_get_ptls_states();
-    maybe_collect(ptls);
+    if (ptls && ptls->safepoint) {
+        maybe_collect(ptls);
+    }
     if (sz < old)
         gc_num.freed += (old - sz);
     else
         gc_num.allocd += (sz - old);
     gc_num.realloc++;
     void *b = realloc(p, sz);
-    if (b == NULL)
+    // If ptls->safepoint is NULL, then let the caller deal with the failed realloc
+    if (b == NULL && ptls && ptls->safepoint)
         jl_throw(jl_memory_exception);
     return b;
 }

src/julia_threads.h

@@ -161,7 +161,7 @@ JL_DLLEXPORT void (jl_cpu_wake)(void);
 
 // gc safepoint and gc states
 // This triggers a SegFault when we are in GC
-// Assign it to a variable to make sure the compiler emit the load
+// Assign it to a variable to make sure the compiler emits the load
 // and to avoid Clang warning for -Wunused-volatile-lvalue
 #define jl_gc_safepoint_(ptls) do {                     \
         jl_signal_fence();                              \