HPCC-12674 RoxieReleaseRow() Avoid locking decrements when possible

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
I would probably make it explicit -
        unsigned rowCount = atomic_read(&header->count)-1;
        unsigned tmpRc;
        if ( (tmpRc=ROWCOUNT(rowCount)) == 0)
            atomic_set(&header->count, rowCount);
        else
        {
            rowCount = atomic_dec_and_read(&header->count);
            tmpRc = ROWCOUNT(rowCount);
        }
        if (tmpRc == 0)
        {