vsh: Accept NULL @list in vshCompleterFilter()

vsh: Accept NULL @list in vshCompleterFilter()

The aim of vshCompleterFilter() is to take a string list and a
prefix and remove all strings from the list that don't have the
desired prefix. The function is used to filter out those strings
returned by a completer callback that don't correspond with
user's (partial) input. For instance, domain name completer
virshDomainNameCompleter() returns all domain names and then
vshCompleterFilter() refines the list so that only domains with
correct prefix of their name are offered to user. This was a
design choice - it allows us to have shorter completers as they
do not have to copy the list filtering over and over.

Having said all of that, it may happen that a completer does not
return anything (e.g. there is no domain in requested state,
virsh is not connected and thus completer exited early, etc.). In
that case, the string list is NULL and vshCompleterFilter() can
simply return early.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>