Do not compare an FD_t < 0

Do not compare an FD_t < 0

FD_t is an abstract type.  Invalid file decriptors are == INVALID_FD.
In most places this module does the right thing.  Fix the last
stragglers where it is testing as less than zero.

Change-Id: I9ae3db8775ce69da070a4ebef5a532e1db035977
Reviewed-on: http://gerrit.openafs.org/3768
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>