Fix crash when saving Preferences and current game doesn't exist

src/PrefsManager.cpp

@@ -370,7 +370,7 @@ void PrefsManager::StoreGamePrefs()
 	ASSERT( !m_sCurrentGame.Get().empty() );
 
 	// save off old values
-	GamePrefs &gp = m_mapGameNameToGamePrefs.at(m_sCurrentGame.Get());
+	GamePrefs &gp = m_mapGameNameToGamePrefs[m_sCurrentGame.Get()];
 	gp.m_sAnnouncer = m_sAnnouncer;
 	gp.m_sTheme = m_sTheme;
 	gp.m_sDefaultModifiers = m_sDefaultModifiers;