file size on non-(Linux/BSD/Windows)-installations

Determining the file size using the exec() method is implemented for Linux, BSD, and Windows. However, on systems matching neither platform name (like SunOS), the fall-through path will return a file size result constituting a zero size instead of an invalid null return value.
Merge pull request #10249 from mroi/patch-1

file size on non-(Linux/BSD/Windows)-installations