Add nbtree page deletion assertion.
Add a documenting assertion that's similar to the nearby assertion added
by commit  This conveys that the entire call to _bt_pagedel()
does no work if it isn't possible to get a descent stack for the initial
scanblkno page.
@@ -1697,6 +1697,7 @@ _bt_pagedel(Relation rel, Buffer leafbuf, TransactionId *oldestBtpoXact)
if (_bt_leftsib_splitflag(rel, leftsib, leafblkno))
{
ReleaseBuffer(leafbuf);
+ Assert(ndeleted == 0);
return ndeleted;
}
