Fix bug in llListSortStrided producing incorrect results sometimes

It used `a = lst[i]` instead of taking the stride index into account (that is, it should have been `a = lst[i+idx]`). Anyway, the exact same value is in b, so use that instead.

Test case by Tapple Gao.

unit_tests/expr.suite/list-funcs-7.lsl

@@ -7,3 +7,4 @@
 + "T7" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, -3, TRUE)
 + "T8" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, -2, TRUE)
 + "T9" + llListSortStrided([1, "C", 3, "A", 2, "B"], 2, -1, TRUE)
++ "T10" + llListSortStrided([1, "b", 0, "c", 1, "a", 0, "b", 1, "c", 0, "a"], 2, 1, TRUE)

unit_tests/expr.suite/list-funcs-7.out

@@ -49,4 +49,17 @@
 , "B"
 , 1
 , "C"
+, "T10"
+, 1
+, "a"
+, 0
+, "a"
+, 0
+, "b"
+, 1
+, "b"
+, 1
+, "c"
+, 0
+, "c"
 ]