codegen: Don't emit error-variable of CatchClause if not used
codegen: Don't emit error-variable of CatchClause if not used

Found by scan-build

GNOME/vala,frida/vala,frida/vala,GNOME/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala