Fix segfault when checking for duplicate notifications with raw icons
Fix segfault when checking for duplicate notifications with raw icons

If a notification has a raw icon, icon is set to NULL which was passed
without checking to strcmp. We don't (yet) support comparing raw icons so
if a raw icon is set, we can safely assume the notification is not a
duplicate.

LukeShu/dunst,LukeShu/dunst,LukeShu/dunst