Simplify lua-esque assignment

`success = success && savesettings();` is now changed to
`success &= savesettings();`. It's bitwise, and I think C++ should have
had a &&= for completeness, but it shouldn't matter here.

desktop_version/src/Game.cpp

@@ -4816,7 +4816,7 @@ bool Game::savestats(const bool stats_only /*= true*/)
 
     if (!stats_only)
     {
-        success = success && savesettings();
+        success &= savesettings();
     }
 
     return success;