codegen: Check vfunc of abstract/virtual methods for NULL before using it
codegen: Check vfunc of abstract/virtual methods for NULL before using it

Fixes https://gitlab.gnome.org/GNOME/vala/issues/153

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