Fix crash due to invalid pid returned in perf sample
Fix crash due to invalid pid returned in perf sample

In 4.14, the perf sample record may contain pid(-1),
numatop needs to consider this case otherwise the crash
will happen.

Signed-off-by: Jin Yao <6c7c7beae62af07f9c01c282ac2af82ec423c630@intel.com>

01org/numatop,ColinIanKing/numatop,ColinIanKing/numatop,intel/numatop,ColinIanKing/numatop,intel/numatop,01org/numatop