mixer: Fix a bogus free() call in the main loop

After a strsep() call, p might not point at the original allocation
anymore.

Reported by: CHERI
Reviewed by: christos
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47266

(cherry picked from commit c22be0b181e954a4cc00ed4f5c7df974f3061c3c)