seed_script_destroy: only unref source_url if it exists
seed_script_destroy: only unref source_url if it exists

Fixes a crash when creating a script with a NULL source_url,
which is documented to be fine.

https://bugzilla.gnome.org/show_bug.cgi?id=702082

danilocesar/seed,danilocesar/seed,danilocesar/seed,danilocesar/seed,danilocesar/seed