Ensure gatherstate->nextreader is properly initialized.

The previously code worked OK as long as a Gather node was never
rescanned, or if it was rescanned, as long as it got at least as
many workers on rescan as it had originally.  But if the number
of workers ever decreased on a rescan, then it could crash.

Andreas Seltenreich

src/backend/executor/nodeGather.c

@@ -172,6 +172,7 @@ ExecGather(GatherState *node)
 			if (pcxt->nworkers_launched > 0)
 			{
 				node->nreaders = 0;
+				node->nextreader = 0;
 				node->reader =
 					palloc(pcxt->nworkers_launched * sizeof(TupleQueueReader *));
 
@@ -334,6 +335,7 @@ gather_readnext(GatherState *gatherstate)
 		CHECK_FOR_INTERRUPTS();
 
 		/* Attempt to read a tuple, but don't block if none is available. */
+		Assert(gatherstate->nextreader < gatherstate->nreaders);
 		reader = gatherstate->reader[gatherstate->nextreader];
 		tup = TupleQueueReaderNext(reader, true, &readerdone);