Only allow null default value for out parameters
Only allow null default value for out parameters

Fixes bug 636096.

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