Fix rebasing mistake in nodeGather.c

The patches committed as 6e71dd7ce9766582da453f493bc371d64977282f
and 3a1f8611f2582df0a16bcd35caed2e1526387643 were developed in
parallel but dependent on each other in a way that I failed to
notice.

This patch to fix the problem was prepared by Amit Kapila.

src/backend/executor/nodeGather.c

@@ -341,7 +341,7 @@ gather_readnext(GatherState *gatherstate)
 			--gatherstate->nreaders;
 			if (gatherstate->nreaders == 0)
 			{
-				ExecShutdownGather(gatherstate);
+				ExecShutdownGatherWorkers(gatherstate);
 				return NULL;
 			}
 			else