Windows fix: do not pass bad params to fstat(), preventing a except. #4097

fstat() is considered a security related libc call on Windows, and when
security related calls get passed unexpected data (like a bad file
descriptor), Windows invokes a process-wide Invalid Parameter Handler
Routine.  The default handler crashes the application (on purpose).
This patch makes sure the descriptor returned by open() is valid
before handing it down to fstat().
Merge branch 'V8_0-branch' of ssh://tonic.cs.wisc.edu/p/condor/repository/CONDOR_SRC into V8_0-branch