Fix INVARIANTS debug code in HWPMC

When HWPMC stops sampling, ps_pmc may be freed before samples
are processed. In such situation treat PMC as stopped.
Add "ifdef" to fix build without INVARIANTS code.

Submitted by: Michal Mazur <mkm@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield, Netgate
Differential revision: https://reviews.freebsd.org/D10912