GValue: Fix cast from nullable GLib.Value
GValue: Fix cast from nullable GLib.Value

Fixes bug 585050.

Jc2k/vala,GNOME/vala,severinh/vala,spidermonkdust/vala,phako/vala,kamanashisroy/aroop,GNOME/vala,tjol/vala-tjol,tjol/vala-tjol,severinh/vala,frida/vala,frida/vala,GNOME/vala,Distrotech/vala,kamanashisroy/aroop,xdave/pvala,frida/vala,frida/vala,severinh/vala,jessevdk/vala,kamanashisroy/aroop,flobrosch/vala-fixes,phako/vala,playya/vala,GNOME/vala,rvu95/vala,playya/vala,jessevdk/vala,spidermonkdust/vala,spidermonkdust/vala,jessevdk/vala,rvu95/vala,rvu95/vala,Jc2k/vala,frida/vala,themiwi/vala,playya/vala,tjol/vala-tjol,Distrotech/vala,themiwi/vala,themiwi/vala,flobrosch/vala-fixes,Distrotech/vala,flobrosch/vala-fixes