Consistently allow reading of messages from a detached shm_mq.

This was intended to work always, but the previous code only allowed
it if at least one message was successfully read by the receiver
before the sender detached the queue.

Report by Petr Jelinek.  Patch by me.

src/backend/storage/ipc/shm_mq.c

@@ -406,7 +406,8 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
 			if (shm_mq_get_sender(mq) == NULL)
 				return SHM_MQ_WOULD_BLOCK;
 		}
-		else if (!shm_mq_wait_internal(mq, &mq->mq_sender, mqh->mqh_handle))
+		else if (!shm_mq_wait_internal(mq, &mq->mq_sender, mqh->mqh_handle)
+				 && shm_mq_get_sender(mq) == NULL)
 		{
 			mq->mq_detached = true;
 			return SHM_MQ_DETACHED;