lib: parser: fix SEGV when Tab hits non-WORD_TKN

lib: parser: fix SEGV when Tab hits non-WORD_TKN

If <Tab> processing finds that there is only 1 candidate, but that
candidate is not a WORD_TKN that we can tab-complete on, the status
would remain at CMD_COMPLETE_FULL_MATCH, but the resulting list of
possible completions is empty.

This then SEGVs in lib/vty.c where it tries to access the first element
of the list, assuming FULL_MATCH always has 1 element there...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Cc: Quentin Young <qlyoung@cumulusnetworks.com>