viced cap fetchdata len to avoid negative

viced cap fetchdata len to avoid negative

when trying to read from a short or zero length file,
it's possible for sanity checking to knock the length below
zero. set a floor at zero.

Reviewed-on: http://gerrit.openafs.org/494
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 529d487d65d8561f5d0a43a4dc71f72b86efd975)
Reviewed-on: http://gerrit.openafs.org/500