[XAM/User] XamUserGetGamerTag - added exception for user_index -1 (0xFF)

(Fixes gamertag in 9199 dash)

src/xenia/kernel/xam/xam_user.cc

@@ -194,7 +194,7 @@ DECLARE_XAM_EXPORT1(XamUserGetName, kUserProfiles, kImplemented);
 
 dword_result_t XamUserGetGamerTag(dword_t user_index, lpwstring_t buffer,
                                   dword_t buffer_len) {
-  if (user_index) {
+  if (user_index && user_index != 0xFF) {
     return X_ERROR_NO_SUCH_USER;
   }