ensure that GetCreditsMessage returns a value

The unhandled case: in Free Play mode and players can't join.  Can this
even happen?

src/ScreenSystemLayer.cpp

@@ -119,8 +119,11 @@ namespace
 			default: // CoinMode_Free
 				if( GAMESTATE->PlayersCanJoin() )
 					return CREDITS_FREE_PLAY.GetValue();
+				// TODO: What should be displayed if players
+				// can't join in free mode?
 			}
 		}
+		return RString();
 	}
 
 };