Tighten up assertion on a local variable

'lineindex' is 0-based, as mentioned in the comments.

Backpatch to v18 where the assertion was added.

Author: ChangAo Chen <cca5507@qq.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/tencent_A84F3C810365BB9BD08442955AE494141907@qq.com
Backpatch-through: 18

src/backend/access/heap/heapam.c

@@ -1077,7 +1077,7 @@ continue_page:
 			ItemId		lpp;
 			OffsetNumber lineoff;
 
-			Assert(lineindex <= scan->rs_ntuples);
+			Assert(lineindex < scan->rs_ntuples);
 			lineoff = scan->rs_vistuples[lineindex];
 			lpp = PageGetItemId(page, lineoff);
 			Assert(ItemIdIsNormal(lpp));