Don't panic in hwpmc when stopping sampling.

When hwpmc stops sampling it will set the pm_state to something other
than PMC_STATE_RUNNING. This means the following sequence can happen:

CPU 0: Enter the interrupt handler
CPU 0: Set the thread TDP_CALLCHAIN pflag
CPU 1: Stop sampling
CPU 0: Call pmc_process_samples, sampling is stopped so clears ps_nsamples
CPU 0: Finishes interrupt processing with the TDP_CALLCHAIN flag set
CPU 0: Call pmc_capture_user_callchain to capture the user call chain
CPU 0: Find all the pmc sample are free so no call chains need to be captured
CPU 0: KASSERT because of this

This fixes the issue by checking if any of the samples have been stopped
and including this in te KASSERT.

PR: 204273
Reviewed by: bz, gnn
Obtained from: ABT Systems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6581