Fix a possible use after free

Found by coverity. The cachedIcon can be deleted
when being inserted into the icon cache (in QCache::insert).
So copy it to icon before trying to insert it into the cache.

Change-Id: I5ed13c0c7ecb8f8f13285ca5d06237493dbea479
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/gui/image/qicon.cpp

@@ -1173,8 +1173,8 @@ QIcon QIcon::fromTheme(const QString &name, const QIcon &fallback)
         QIconEngine * const engine = platformTheme ? platformTheme->createIconEngine(name)
                                                    : new QIconLoaderEngine(name);
         QIcon *cachedIcon  = new QIcon(engine);
-        qtIconCache()->insert(name, cachedIcon);
         icon = *cachedIcon;
+        qtIconCache()->insert(name, cachedIcon);
     }
 
     // Note the qapp check is to allow lazy loading of static icons