A protobuf message will be corrupted in the following scenario:
  1. Use LITE_RUNTIME.
  2. Have an optional enum field following some other field.
  3. Update protocol by adding new values to the enum.
  4. Have an old client parse and serialize a message having enum field
      set to a value the client does not understand.
  5. Field preceeding the enum is now corrupted.

The bug is due to the fact that optimized fallthrough in parser code
does not update variablle 'tag' when jumping to the parser code for the
next field.