Silence another _bt_check_unique compiler warning.

Per complaint from Tom Lane

Discussion: https://postgr.es/m/1922884.1617909599@sss.pgh.pa.us

src/backend/access/nbtree/nbtinsert.c

@@ -408,7 +408,7 @@ _bt_check_unique(Relation rel, BTInsertState insertstate, Relation heapRel,
 {
 	IndexTuple	itup = insertstate->itup;
 	IndexTuple	curitup = NULL;
-	ItemId		curitemid;
+	ItemId		curitemid = NULL;
 	BTScanInsert itup_key = insertstate->itup_key;
 	SnapshotData SnapshotDirty;
 	OffsetNumber offset;