GTK-style - fix non-standard usage of QHash-iterator

Even if it currently works it is highly non standard to
modify (incl. free/delete) the key for the iterator and then
use it afterwards.

The current code prevents a merge of a patch that uses the
key to check if the iterator is valid.

Change-Id: Ia89553ea4ba53e24488a6bfe06b8d7d0e351c1ea
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>

src/widgets/styles/qgtkstyle_p.cpp

@@ -756,8 +756,9 @@ void QGtkStylePrivate::removeWidgetFromMap(const QHashableLatin1Literal &path)
     WidgetMap *map = gtkWidgetMap();
     WidgetMap::iterator it = map->find(path);
     if (it != map->end()) {
-        free(const_cast<char *>(it.key().data()));
+        char* keyData = const_cast<char *>(it.key().data());
         map->erase(it);
+        free(keyData);
     }
 }