Fix for Clang 6.
Handpicked from upstream commit 8660d49aae9e4e357d750ffd1dbb7ef850b2b76d.

Index: src/PrefsManager.cpp
--- src/PrefsManager.cpp.orig
+++ src/PrefsManager.cpp
@@ -374,7 +374,7 @@ void PrefsManager::StoreGamePrefs()
 	ASSERT( !m_sCurrentGame.Get().empty() );
 
 	// save off old values
-	GamePrefs &gp = m_mapGameNameToGamePrefs[m_sCurrentGame];
+	GamePrefs &gp = m_mapGameNameToGamePrefs[m_sCurrentGame.ToString()];
 	gp.m_sAnnouncer = m_sAnnouncer;
 	gp.m_sTheme = m_sTheme;
 	gp.m_sDefaultModifiers = m_sDefaultModifiers;
