Simplify the logic in mrb_realloc; ref #3679

src/gc.c

@@ -215,7 +215,8 @@ mrb_realloc(mrb_state *mrb, void *p, size_t len)
   void *p2;
 
   p2 = mrb_realloc_simple(mrb, p, len);
-  if (!p2 && len) {
+  if (len == 0) return p2;
+  if (p2 == NULL) {
     if (mrb->gc.out_of_memory) {
       mrb_exc_raise(mrb, mrb_obj_value(mrb->nomem_err));
       /* mrb_panic(mrb); */