Fix strings.index_any on small strings

tests/core/strings/test_core_strings.odin

@@ -0,0 +1,29 @@
+package test_core_image
+
+import "core:strings"
+import "core:testing"
+
+@test
+test_index_any_small_string_not_found :: proc(t: ^testing.T) {
+    index := strings.index_any(".", "/:\"")
+    testing.log(t, index)
+    testing.expect(t, index == -1, "index_any should be negative")
+}
+
+@test
+test_index_any_larger_string_not_found :: proc(t: ^testing.T) {
+    index := strings.index_any("aaaaaaaa.aaaaaaaa", "/:\"")
+    testing.expect(t, index == -1, "index_any should be negative")
+}
+
+@test
+test_index_any_small_string_found :: proc(t: ^testing.T) {
+    index := strings.index_any(".", "/:.\"")
+    testing.expect(t, index == 0, "index_any should be 0")
+}
+
+@test
+test_index_any_larger_string_found :: proc(t: ^testing.T) {
+    index := strings.index_any("aaaaaaaa:aaaaaaaa", "/:\"")
+    testing.expect(t, index == 8, "index_any should be 8")
+}