codegen: Fix double free with GValue boxing
codegen: Fix double free with GValue boxing

Fixes bug 631356.

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