LU-6558 ptlrpc: handle NULL export in nrs_orr_key_fill()

LU-6558 ptlrpc: handle NULL export in nrs_orr_key_fill()

In nrs_orr_key_fill() the request export may be NULL so avoid
dereferencing it.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: I7f0a3a898586eb49ae7c1fadd9118759179280b9
Reviewed-on: http://review.whamcloud.com/14737
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>