LU-7636 gss: avoid useless sec debug log flooding

LU-7636 gss: avoid useless sec debug log flooding

Avoid useless sec debug log flooding by printing gss context deletion
messages only when context is not null.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I9a93813c52c6edadac435e5ba53ed292b81d6566
Reviewed-on: http://review.whamcloud.com/17869
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>