Assert Ruby object in rb_gc_location

rb_gc_location doesn't check that the object is actually a Ruby object
and only checks if the object looks like a T_MOVED. This may have unexpected
outcomes if the object is not a Ruby object (e.g. a piece of malloc memory
may be corrupted).

gc.c

@@ -2427,6 +2427,8 @@ gc_location_internal(void *objspace, VALUE value)
         return value;
     }
 
+    GC_ASSERT(rb_gc_impl_pointer_to_heap_p(objspace, (void *)value));
+
     return rb_gc_impl_location(objspace, value);
 }