Fix possible failure in parallel index build.

Report and proposed fix by David Rowley, put in patch form by
Peter Geoghegan.

Discussion: http://postgr.es/m/CAKJS1f91kq1wfYR8rnRRfKtxyhU2woEA+=whd640UxMyU+O0EQ@mail.gmail.com

src/backend/catalog/index.c

@@ -4064,8 +4064,7 @@ RemoveReindexPending(Oid indexOid)
 static void
 ResetReindexPending(void)
 {
-	if (IsInParallelMode())
-		elog(ERROR, "cannot modify reindex state during a parallel operation");
+	/* This may be called in leader error path */
 	pendingReindexedIndexes = NIL;
 }