codegen: Guard target_value to be non-null in visit_expression
codegen: Guard target_value to be non-null in visit_expression

Fixes bug 657499.

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