Fix ReinitializeParallelDSM to tolerate finding no error queues.

Commit d466335064 changed things so
that shm_toc_lookup would fail with an error rather than silently
returning NULL in the hope that such failures would be reported
in a useful way rather than via a system crash.  However, it
overlooked the fact that the lookup of PARALLEL_KEY_ERROR_QUEUE
in ReinitializeParallelDSM is expected to fail when no DSM segment
was created in the first place; in that case, we end up with a
backend-private memory segment that still contains an entry for
PARALLEL_KEY_FIXED but no others.  Consequently a benign failure
to initialize parallelism can escalate into an elog(ERROR);
repair.

Discussion: http://postgr.es/m/CA+Tgmob8LFw55DzH1QEREpBEA9RJ_W_amhBFCVZ6WMwUhVpOqg@mail.gmail.com

src/backend/access/transam/parallel.c

@@ -428,9 +428,10 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 	fps = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_FIXED, false);
 	fps->last_xlog_end = 0;
 
-	/* Recreate error queues. */
+	/* Recreate error queues (if they exist). */
 	error_queue_space =
-		shm_toc_lookup(pcxt->toc, PARALLEL_KEY_ERROR_QUEUE, false);
+		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)
 	{
 		char	   *start;