src: fix abort when taking a heap snapshot

Remove an erroneous CHECK that asserted the persistent object's internal
field pointer still pointed to a valid object.  If ClearWrap() has been
called, the field pointer equals nullptr and that is expected behavior.

PR-URL: #18898
Fixes: #18256
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matheus Marchini <matheus@sthima.com>

src/async_wrap.cc

@@ -124,7 +124,7 @@ RetainedObjectInfo* WrapperInfo(uint16_t class_id, Local<Value> wrapper) {
   CHECK_GT(object->InternalFieldCount(), 0);
 
   AsyncWrap* wrap = Unwrap<AsyncWrap>(object);
-  CHECK_NE(nullptr, wrap);
+  if (wrap == nullptr) return nullptr;  // ClearWrap() already called.
 
   return new RetainedAsyncInfo(class_id, wrap);
 }