RetroAchievements: Avoid uninitialized memory access

Avoid creating and then destroying a leaderboard list when game is null,
as doing so causes an access to uninitialized memory due to a bug in
rcheevos.

This can be triggered by starting a game with an invalid or expired
login token.

Source/Core/Core/AchievementManager.cpp

@@ -1027,6 +1027,9 @@ void AchievementManager::LoadGameCallback(int result, const char* error_message,
                     OSD::Color::RED);
   }
 
+  if (game == nullptr)
+    return;
+
   rc_client_set_read_memory_function(instance.m_client, MemoryPeeker);
   instance.FetchGameBadges();
   instance.m_system.store(&Core::System::GetInstance(), std::memory_order_release);