Add nbtree page deletion assertion.

Add a documenting assertion that's similar to the nearby assertion added
by commit cd8c73a3.  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.

src/backend/access/nbtree/nbtpage.c

@@ -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;
 				}