afsmonitor: Check correct variable is non-NULL

Before freeing curr_fsData, check that it is non-NULL, rather than
checking prev_fsData in error.

Caught by coverity (#985289)

Reviewed-on: http://gerrit.openafs.org/9284
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 5a61e826fa0de6a5987f06257199476f79d2473a)

Change-Id: Ie63e90f8873f4fbe8afc7abee160a090920add4d
Reviewed-on: http://gerrit.openafs.org/11015
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>