s3:dbwrap_ctdb: fix logic error in pull_newest_from_marshall_buffer().

s3:dbwrap_ctdb: fix logic error in pull_newest_from_marshall_buffer().

The logic bug was that if a record was found in the marshall buffer,
then always the ctdb header of tha last record in the marshall buffer
was returned, and not the ctdb header of the last occurrence of the
requested record.

This is fixed by introducing an additional temporary variable.

Michael