Double check size of read protobuf memory

Speculative fix for issue 980094. Ensure that the read protobuf memory
has a size and is not a nullptr before trying to have the ProtoBuf
library parse it.

Bug: 980094
Change-Id: Idaf47884e82eff8c0850a6ec27fc8d452c10e622
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1768762
Auto-Submit: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690236}

third_party/blink/common/font_unique_name_lookup/font_table_persistence.cc

@@ -48,7 +48,8 @@ bool LoadFromFile(base::FilePath file_path,
   const char* proto_data = nullptr;
   int proto_length = 0;
 
-  if (!pickle_iterator.ReadData(&proto_data, &proto_length)) {
+  if (!pickle_iterator.ReadData(&proto_data, &proto_length) || !proto_data ||
+      proto_length <= 0) {
     return false;
   }