glib-2.0: Fix some issues for the experimental non-null checks
glib-2.0: Fix some issues for the experimental non-null checks

Fixes bug 651841.

jessevdk/vala,spidermonkdust/vala,rvu95/vala,severinh/vala,playya/vala,jessevdk/vala,flobrosch/vala-fixes,Distrotech/vala,Distrotech/vala,GNOME/vala,tjol/vala-tjol,rvu95/vala,xdave/pvala,frida/vala,spidermonkdust/vala,frida/vala,frida/vala,frida/vala,Distrotech/vala,flobrosch/vala-fixes,playya/vala,severinh/vala,GNOME/vala,severinh/vala,jessevdk/vala,playya/vala,flobrosch/vala-fixes,GNOME/vala,tjol/vala-tjol,GNOME/vala,frida/vala,rvu95/vala,tjol/vala-tjol,spidermonkdust/vala