py: Handle case of slice start > stop in common sequence function.

py/objstr.c

@@ -355,9 +355,6 @@ STATIC mp_obj_t str_subscr(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
             if (!mp_seq_get_fast_slice_indexes(self_len, index, &start, &stop)) {
                 assert(0);
             }
-            if (start >= stop) {
-                return MP_OBJ_NEW_QSTR(MP_QSTR_);
-            }
             return str_new(type, self_data + start, stop - start);
         }
 #endif

py/sequence.c

@@ -88,6 +88,12 @@ bool mp_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, machine_u
     } else if (stop > len) {
         stop = len;
     }
+
+    // CPython returns empty sequence in such case, or point for assignment is at start
+    if (start > stop) {
+        stop = start;
+    }
+
     *begin = start;
     *end = stop;
     return true;