vshCommandOpt: Allow caller avoiding assert()

vshCommandOpt: Allow caller avoiding assert()

In the future, completer callbacks will receive partially parsed
command (and thus possibly incomplete). However, we still want
them to use command options fetching APIs we already have (e.g.
vshCommandOpt*()) and at the same time don't report any errors
(nor call any asserts).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>