Don't dump core when destroying an unused ParallelContext.

If a transaction or subtransaction creates a ParallelContext but ends
without calling InitializeParallelDSM, the previous code would
seg fault.  Fix that.

src/backend/access/transam/parallel.c

@@ -513,6 +513,8 @@ DestroyParallelContext(ParallelContext *pcxt)
 	dlist_delete(&pcxt->node);
 
 	/* Kill each worker in turn, and forget their error queues. */
+	if (pcxt->worker != NULL)
+	{
 		for (i = 0; i < pcxt->nworkers; ++i)
 		{
 			if (pcxt->worker[i].bgwhandle != NULL)
@@ -523,6 +525,7 @@ DestroyParallelContext(ParallelContext *pcxt)
 				pcxt->worker[i].error_mqh = NULL;
 			}
 		}
+	}
 
 	/*
 	 * If we have allocated a shared memory segment, detach it.  This will
@@ -550,7 +553,7 @@ DestroyParallelContext(ParallelContext *pcxt)
 	{
 		BgwHandleStatus status;
 
-		if (pcxt->worker[i].bgwhandle == NULL)
+		if (pcxt->worker == NULL || pcxt->worker[i].bgwhandle == NULL)
 			continue;
 
 		/*