[sancov] Ignore PC samples with value 0
The sancov runtime for the (Fuchsia) Zircon kernel delivers results
in the standard format, but as the full array of possible samples
with 0 in uncovered slots. That runtime delivers "live" data and
has no final "export" pass to compactify out the uncovered slots,
and it seems silly to require another offline tool just for that.
Patch By: mcgrathr
Differential Revision: https://reviews.llvm.org/D63695