server/admin: Brown-paper-bag fix.

Commit 165efde7 changed a_resolve to not require a port number.
Unfortunately, I botched it and wrote a test for av[i + 1] as
av[i + i].  Result with i = 0: very different.

While investigating this bug, I became nervous about the number of
is-av[i]-null tests going on when str_qsplit doesn't actually guarantee
to leave a null terminator behind if it uses all the array slots.  So
I've allocated an extra slot and zeroed it explicitly.