py/map: Replace always-false condition with assertion.

py/map.c

@@ -148,11 +148,8 @@ STATIC void mp_map_rehash(mp_map_t *map) {
 // MP_MAP_LOOKUP_REMOVE_IF_FOUND behaviour:
 //  - returns NULL if not found, else the slot if was found in with key null and value non-null
 mp_map_elem_t *mp_map_lookup(mp_map_t *map, mp_obj_t index, mp_map_lookup_kind_t lookup_kind) {
-
-    if (map->is_fixed && lookup_kind != MP_MAP_LOOKUP) {
-        // can't add/remove from a fixed array
-        return NULL;
-    }
+    // If the map is a fixed array then we must only be called for a lookup
+    assert(!map->is_fixed || lookup_kind == MP_MAP_LOOKUP);
 
     // Work out if we can compare just pointers
     bool compare_only_ptrs = map->all_keys_are_qstrs;