Don't create objects during GC

If we crash during GC, allocating new objects in the segv handler can
cause an infinite loop.  This commit is to avoid creating new objects in
the crash handler

vm_dump.c

@@ -1048,7 +1048,7 @@ rb_vm_bugreport(const void *ctx)
 	fprintf(stderr, "-- Other runtime information "
 		"-----------------------------------------------\n\n");
     }
-    if (vm) {
+    if (vm && !rb_during_gc()) {
 	int i;
 	VALUE name;
 	long len;