Fixed username using non-const variant

src/modb_users.c

@@ -317,7 +317,7 @@ int64_t modbUserCreate(stored_conn *sconn, modb_ref *modb, struct user_t *user)
   if (user->email_c == 0) {
     user->email_c = user->email;
   }
-  if (user->username == 0) {
+  if (user->username_c == 0) {
     user->username_c = user->username;
   }