Fix possible use of cachedIcon after free
Fix possible use of cachedIcon after free

QCache::insert() may free cachedIcon.
Avoid dereferencing a possibly freed pointer (cachedIcon)

Found by Coverity Scan. See:
https://scan.coverity.com/projects/libqtxdg-libqtxdg

libqtxdg/libqtxdg,libqtxdg/libqtxdg,luis-pereira/libqtxdg,luis-pereira/libqtxdg,lxde/libqtxdg,lxde/libqtxdg