network.subr: Fix infinite loop

When setting up carp tunnel, using a password consisting of only the
characters used as hexadecimal characters, i.e. abc-def, there will be
an infinite loop in the shell function ifalias_af_common_handler().
To circumvent this we test for " pass ".

PR: 268378
Reported by: jyoung15@gmail.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37748