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.

lslopt/lslbasefuncs.py

@@ -1704,8 +1704,8 @@ def llListSort(lst, stride, asc, idx=0):
                 # swap
                 lst[i:i+stride],lst[j:j+stride] = lst[j:j+stride],lst[i:i+stride]
                 # Re-cache
-                a = lst[i]
-                ta = type(a)
+                a = b
+                ta = tb
                 if ta == Vector:
                     a = v2f(a)
                     a = a[0]*a[0] + a[1]*a[1] + a[2]*a[2]

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"
 ]