butm/file_tm.c: check for null pointer before dereferencing

Move the null pointer check to before the pointer is dereferenced.

Reviewed-on: http://gerrit.openafs.org/8882
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 4d6967e0b0a0628deb1dd9d210de84779d41f856)

Change-Id: Ib6c9308170b1242ea207f5380275d51b1514028d
Reviewed-on: http://gerrit.openafs.org/9515
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>