Fix popen return value check

[illiav-j]

Fixes the error when trying to build on the latest OpenBMC:

| …/src/cpu_info.cpp: In member function 'bool CpuInfo::getNumberOfCpu()':
| …/src/cpu_info.cpp:530:14: error: ordered comparison of pointer with integer zero ('FILE*' and 'int')
|   530 |        if(pf > 0)
|       |           ~~~^~~

This change has not been tested.