Fix dialyzer issue for sub-messages only in oneof
Fix dialyzer issue for sub-messages only in oneof

The following proto file would cause two unnecessary extra function
clauses for the internal merge function for merging the 'S' msg,
and Dialyzer rightfully found that.

    message S {
      required int32 i = 1;
    }

    message Test {
      oneof u {
        S a = 10;
      };
    }

mmmries/gpb