Consolidate WindowsWmicFinder

[theofidry]

• Move the popen function check instead of relying on the CpuCoreCounter's proc_open check.
• Return null directly if is not on Windows (hence the command will not work); this was previously done in CpuCoreCounter but it more belongs to the finder as it's really a micro-optimization since the constant defined check is faster than doing popen
• Refactor WindowsWmicFinder::find() for better readability
• Handle potential fgets false value returned