Fix attach-related race condition in shm_mq_send_bytes.
Fix attach-related race condition in shm_mq_send_bytes.

Spotted by Antonin Houska.

adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb