docs: Add vfunc NULL checks to GObject how-to examples
docs: Add vfunc NULL checks to GObject how-to examples

Not setting a pure vfunc is a programmer error, so can be handled with a
g_return_if_fail() rather than needing a g_warning().

https://bugzilla.gnome.org/show_bug.cgi?id=744060

tchakabam/glib,gale320/glib,johne53/MB3Glib,cention-sany/glib,johne53/MB3Glib,ieei/glib,Distrotech/glib,cention-sany/glib,johne53/MB3Glib,johne53/MB3Glib,tchakabam/glib,mzabaluev/glib,ieei/glib,ieei/glib,cention-sany/glib,cention-sany/glib,ieei/glib,gale320/glib,ieei/glib,gale320/glib,mzabaluev/glib,mzabaluev/glib,gale320/glib,MathieuDuponchelle/glib,johne53/MB3Glib,Distrotech/glib,gale320/glib,tchakabam/glib,mzabaluev/glib,endlessm/glib,tchakabam/glib,MathieuDuponchelle/glib,Distrotech/glib,mzabaluev/glib,MathieuDuponchelle/glib,Distrotech/glib,tchakabam/glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,cention-sany/glib,endlessm/glib,Distrotech/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib