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.

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>
(cherry picked from commit 299e942ae98dfea3a4804c724e5d440715974e0c)

Change-Id: I8f676c45794ecf53af65e9b3089c7b2d075089bb
Reviewed-on: http://gerrit.openafs.org/3871
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>