py/objexcept: Fix hash of exc str created in mp_obj_new_exception_msg.

py/objexcept.c

@@ -348,9 +348,9 @@ mp_obj_t mp_obj_new_exception_msg(const mp_obj_type_t *exc_type, const char *msg
 
     // Create the string object and call mp_obj_exception_make_new to create the exception
     o_str->base.type = &mp_type_str;
-    o_str->hash = qstr_compute_hash(o_str->data, o_str->len);
     o_str->len = strlen(msg);
     o_str->data = (const byte*)msg;
+    o_str->hash = qstr_compute_hash(o_str->data, o_str->len);
     mp_obj_t arg = MP_OBJ_FROM_PTR(o_str);
     return mp_obj_exception_make_new(exc_type, 1, 0, &arg);
 }