Fix wal_consistency_checking nbtree bug.

wal_consistency_checking indicated an inconsistency in certain cases
involving nbtree page deletion.  The underlying issue is that there was
a minor difference between the page image produced after a REDO routine
ran and the corresponding page image following original execution.

This harmless inconsistency has been around forever.  We more or less
expect total consistency among even deleted nbtree pages these days,
though, so this won't do anymore.

To fix, tweak the REDO routine to match original execution.

Oversight in commit f47b5e13.

src/backend/access/nbtree/nbtxlog.c

@@ -824,6 +824,8 @@ btree_xlog_unlink_page(uint8 info, XLogReaderState *record)
 	pageop->btpo_next = rightsib;
 	pageop->btpo.xact = xlrec->btpo_xact;
 	pageop->btpo_flags = BTP_DELETED;
+	if (!BlockNumberIsValid(xlrec->topparent))
+		pageop->btpo_flags |= BTP_LEAF;
 	pageop->btpo_cycleid = 0;
 
 	PageSetLSN(page, lsn);