Silence another compiler warning in nbtinsert.c.

Per complaint from Álvaro Herrera.

Author: petergeoghegan

src/backend/access/nbtree/nbtinsert.c

@@ -1064,7 +1064,7 @@ _bt_insertonpg(Relation rel,
 	Page		page;
 	BTPageOpaque lpageop;
 	Size		itemsz;
-	IndexTuple	oposting;
+	IndexTuple	oposting = NULL;
 	IndexTuple	origitup = NULL;
 	IndexTuple	nposting = NULL;