py/objstr: In find/rfind, don't crash when end < start.

py/objstr.c

@@ -699,8 +699,13 @@ STATIC mp_obj_t str_finder(size_t n_args, const mp_obj_t *args, int direction, b
         end = str_index_to_ptr(self_type, haystack, haystack_len, args[3], true);
     }
 
+    if (end < start) {
+        goto out_error;
+    }
+
     const byte *p = find_subbytes(start, end - start, needle, needle_len, direction);
     if (p == NULL) {
+    out_error:
         // not found
         if (is_index) {
             mp_raise_ValueError("substring not found");