xdr: Len can never be negative

The len parameter to xdr_len is unsigned, so can never be negative.
Don't bother testing to see if it is negative, so that clang likes us
again.

Reviewed-on: http://gerrit.openafs.org/7089
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit f1842bcb87c84e8862ca521f491215e42ce1ee2a)

Change-Id: I1cf030b8e9f79bdf867bcaecfd59d8ade7fc8c25
Reviewed-on: http://gerrit.openafs.org/10734
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>