objstr: Avoid variable shadowing.

py/objstr.c

@@ -518,12 +518,12 @@ STATIC inline mp_obj_t str_split_internal(mp_uint_t n_args, const mp_obj_t *args
                 }
                 s++;
             }
-            mp_uint_t len = s - start;
+            mp_uint_t sub_len = s - start;
             if (MP_LIKELY(!(len == 0 && s == top && (type && SPLITLINES)))) {
                 if (start + len != top && (type & KEEP)) {
-                    len++;
+                    sub_len++;
                 }
-                mp_obj_list_append(res, mp_obj_new_str_of_type(self_type, start, len));
+                mp_obj_list_append(res, mp_obj_new_str_of_type(self_type, start, sub_len));
             }
             if (s >= top) {
                 break;