pmc(8): zero-initialize the event descriptor in stat mode

[afscoelho_gmail.com]

Summary

cmd_pmc_stat allocated the event descriptor with malloc and then set
only some of its fields, so the rest held whatever was on the heap.
Use calloc.

This is an independent fix. It does not depend on the PMU grouping
work, and can be committed on its own.

Sponsored by: AMD

Signed-off-by: Andre Silva <[e-mail]>