validate resource name format before PopFirst in bundle parsing

Author: alhudz

Firestore/core/src/bundle/bundle_serializer.cc

@@ -400,7 +400,8 @@ ResourcePath BundleSerializer::DecodeName(JsonReader& reader,
   }
   auto path =
       ResourcePath::FromString(document_name.get_ref<const std::string&>());
-  if (!rpc_serializer_.IsLocalResourceName(path)) {
+  if (!rpc_serializer_.IsLocalResourceName(path) || path.size() <= 4 ||
+      path[4] != "documents") {
     reader.Fail("Resource name is not valid for current instance: " +
                 path.CanonicalString());
     return {};

Firestore/core/src/remote/serializer.cc

@@ -1521,7 +1521,8 @@ bool Serializer::IsLocalResourceName(const ResourcePath& path) const {
 
 bool Serializer::IsLocalDocumentKey(absl::string_view path) const {
   auto resource = ResourcePath::FromStringView(path);
-  return IsLocalResourceName(resource) &&
+  return IsLocalResourceName(resource) && resource.size() > 4 &&
+         resource[4] == "documents" &&
          DocumentKey::IsDocumentKey(resource.PopFirst(5));
 }