LU-4629 ldlm: fix NULL pointer dereference

LU-4629 ldlm: fix NULL pointer dereference

Pointer '*exp' returned from call to function 'class_conn2export'
at line 523 may be NULL and may be dereferenced at line 543.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I084ae2ae76861aef4fd93477551bc9d0dd5f4478
Reviewed-on: http://review.whamcloud.com/9323
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>