Nullify internal_ pointer when DataSnapshot is delete.

This might not fix b/133190790 but just in case DataSnapshot can still be accessed after deletion.

PiperOrigin-RevId: 249670825

Author: chkuang-g

database/src/common/data_snapshot.cc

@@ -91,9 +91,9 @@ DataSnapshot& DataSnapshot::operator=(DataSnapshot&& snapshot) {
 
 DataSnapshot::~DataSnapshot() {
   CleanupFnDataSnapshot::Unregister(this, internal_);
-  if (internal_) {
-    delete internal_;
-  }
+
+  delete internal_;
+  internal_ = nullptr;
 }
 
 bool DataSnapshot::exists() const { return internal_ && internal_->Exists(); }