hwpmc_x86: Fix NULL deref when loading on unsupported hardware

[bnovkov]

Summary

The pmc_md_{intialize, finalize} routines rely on a machine-dependent
structure to register the appropriate PMC interrupt handler. However,
the vendor-specific routines that allocate this structure may return
NULL for unsupported hardware, leading to a panic when the hwpmc module
gets loaded. This patch adds additional checks that fix this issue.

[kib]

sys/dev/hwpmc/hwpmc_x86.c

233

242

Remove these two lines

243

[bnovkov]

Address @kib 's comments.