void add(int i, FieldDescriptor field, ProtoToHiveConvertor convertor) {
      fields[i] = field;
      convertors[i] = convertor;
    }
Convert the given writable to a message.
@param writable The writable object containing the message.
@return The converted message object.
@throws SerDeException
ql/src/java/org/apache/hadoop/hive/ql/io/protobuf/ProtobufSerDe.java