ip_htable: fix -Wtautological-pointer-compare warnings
This also fixes a few other trivial -Wunused-but-set-variable warnings.
Pull Request: https://github.com/freebsd/freebsd-src/pull/1677
MFC after: 1 month
(cherry picked from commit e4c864e5550544e5ff7e395309c5098d84058403)