libalias: replace placeholder with static constant

The field nullAddress in struct libalias is never set and never used.
It exists as a placeholder for an unused argument only.

Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30253

(cherry picked from commit 189f8eea138a78b09c9f19114b1362b0df1cf87d)
(cherry picked from commit b03a41befeaf17ef25da96fc7bc2dc19c9a6b253)