pgsql: shm_mq: Repair breakage from previous commit.

shm_mq: Repair breakage from previous commit.

If the counterparty writes some data into the queue and then detaches,
it's wrong to return SHM_MQ_DETACHED right away.  If we do that, we
fail to read whatever was written.

Branch
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/44390e30f8531906ed142336f84f172b93073038

Modified Files
--------------
src/backend/storage/ipc/shm_mq.c |    6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)