Use lfirst_int in cmp_list_len_contents_asc

The function added in be45be9c is comparing integer lists (IntList) by
length and contents, but there were two bugs.  Firstly, it used intVal()
to extract the value, but that's for Value nodes, not for extracting int
values from IntList.  Secondly, it called it directly on the ListCell,
without doing lfirst().  So just do lfirst_int() instead.

Interestingly enough, this did not cause any crashes on the buildfarm,
but valgrind rightfully complained about it.

Discussion: https://postgr.es/m/bf3805a8-d7d1-ae61-fece-761b7ff41ecc@postgresfriends.org

src/backend/parser/parse_agg.c

@@ -1750,8 +1750,8 @@ cmp_list_len_contents_asc(const ListCell *a, const ListCell *b)
 
 		forboth(lca, la, lcb, lb)
 		{
-			int		va = intVal(lca);
-			int		vb = intVal(lcb);
+			int		va = lfirst_int(lca);
+			int		vb = lfirst_int(lcb);
 			if (va > vb)
 				return 1;
 			if (va < vb)