Be more wary about shm_toc_lookup failure.

Commit 445dbd82 basically missed the point of commit d4663350,
which was that we shouldn't allow shm_toc_lookup() failure to lead
to a core dump or assertion crash, because the odds of such a
failure should never be considered negligible.  It's correct that
we can't expect the PARALLEL_KEY_ERROR_QUEUE TOC entry to be there
if we have no workers.  But if we have no workers, we're not going
to do anything in this function with the lookup result anyway,
so let's just skip it.  That lets the code use the easy-to-prove-safe
noError=false case, rather than anything requiring effort to review.

Back-patch to v10, like the previous commit.

Discussion: https://postgr.es/m/3647.1517601675@sss.pgh.pa.us

src/backend/access/transam/parallel.c

@@ -434,8 +434,6 @@ void
 ReinitializeParallelDSM(ParallelContext *pcxt)
 {
 	FixedParallelState *fps;
-	char	   *error_queue_space;
-	int			i;
 
 	/* Wait for any old workers to exit. */
 	if (pcxt->nworkers_launched > 0)
@@ -456,18 +454,23 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 	fps->last_xlog_end = 0;
 
 	/* Recreate error queues (if they exist). */
-	error_queue_space =
-		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, true);
-	Assert(pcxt->nworkers == 0 || error_queue_space != NULL);
-	for (i = 0; i < pcxt->nworkers; ++i)
+	if (pcxt->nworkers > 0)
 	{
-		char	   *start;
-		shm_mq	   *mq;
+		char	   *error_queue_space;
+		int			i;
 
-		start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
-		mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
-		shm_mq_set_receiver(mq, MyProc);
-		pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+		error_queue_space =
+			shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
+		for (i = 0; i < pcxt->nworkers; ++i)
+		{
+			char	   *start;
+			shm_mq	   *mq;
+
+			start = error_queue_space + i * PARALLEL_ERROR_QUEUE_SIZE;
+			mq = shm_mq_create(start, PARALLEL_ERROR_QUEUE_SIZE);
+			shm_mq_set_receiver(mq, MyProc);
+			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
+		}
 	}
 }