lib: cli: fix IPv6 address partial matching

lib: cli: fix IPv6 address partial matching

A partially-entered IPv6 address would never return a "partly_match",
meaning some possible completions weren't listed by the matcher.

This specifically breaks autocompleting BGP IPv6 neighbor addresses.
Before:
aegaeon# show ip bg ne 2001:<?>
  WORD      Neighbor on BGP configured interface

After:
aegaeon# show ip bg ne 2001:<?>
  WORD      Neighbor on BGP configured interface
  X:X::X:X  Neighbor to display information about
      2001:db8::2

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>