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>