LU-6507 obdclass: Fix possible NULL pointer dereference

LU-6507 obdclass: Fix possible NULL pointer dereference

The imp->imp_connection really could be NULL, we better check for it
before dereferencing it in taht call to libcfs_nid2str_r.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I16212eb6eb195eb197683606a4e4709810ff708f
Reviewed-on: http://review.whamcloud.com/14808
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>