glib: fix IntPtr.Zero being dereferenced in GLib.Value.ToBoxed()
glib: fix IntPtr.Zero being dereferenced in GLib.Value.ToBoxed()

orion75/gtk-sharp,akrisiun/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,sillsdev/gtk-sharp,antoniusriha/gtk-sharp,antoniusriha/gtk-sharp,akrisiun/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,antoniusriha/gtk-sharp,sillsdev/gtk-sharp,akrisiun/gtk-sharp,orion75/gtk-sharp,akrisiun/gtk-sharp,Gankov/gtk-sharp,orion75/gtk-sharp,openmedicus/gtk-sharp,antoniusriha/gtk-sharp,openmedicus/gtk-sharp,sillsdev/gtk-sharp,akrisiun/gtk-sharp,orion75/gtk-sharp,Gankov/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,Gankov/gtk-sharp,openmedicus/gtk-sharp,orion75/gtk-sharp,sillsdev/gtk-sharp