subcommand: Avoid referencing possibly undefined $ret.

subcommand: Avoid referencing possibly undefined $ret.

If a subcommand does not set $ret (like the minimal "hello" example
does), and sets the nounset bash option (set -u), gsu aborts at
the attempt to reference $ret. Fix this flaw by setting $ret to
$GSU_SUCCESS prior to calling the subcommand.

Reported by Congmao Wang.