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)

Change-Id: I8fb74ce0c4e447de65924f2afb6075213d839760
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>