nbtree page deletion: Add leaftopparent assertion.

Add documenting assertion.  This makes it easier to follow how we
maintain the top parent link in target subtree's half-dead/leaf level
page.

src/backend/access/nbtree/nbtpage.c

@@ -2458,6 +2458,9 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, BlockNumber scanblkno,
 			leaftopparent = InvalidBlockNumber;
 	}
 
+	/* No leaftopparent for level 0 (leaf page) or level 1 target */
+	Assert(!BlockNumberIsValid(leaftopparent) || targetlevel > 1);
+
 	/*
 	 * And next write-lock the (current) right sibling.
 	 */

src/backend/access/nbtree/nbtxlog.c

@@ -818,7 +818,7 @@ btree_xlog_unlink_page(uint8 info, XLogReaderState *record)
 	safexid = xlrec->safexid;
 
 	/* No leaftopparent for level 0 (leaf page) or level 1 target */
-	Assert(xlrec->leaftopparent == InvalidBlockNumber || level > 1);
+	Assert(!BlockNumberIsValid(xlrec->leaftopparent) || level > 1);
 
 	/*
 	 * In normal operation, we would lock all the pages this WAL record