codegen: Cast vfunc pointer assigns of abstract overrides
codegen: Cast vfunc pointer assigns of abstract overrides

https://bugzilla.gnome.org/show_bug.cgi?id=777958

GNOME/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,frida/vala,frida/vala