LU-7416 osp: check rq_repmsg in osp_request_commit_cb

LU-7416 osp: check rq_repmsg in osp_request_commit_cb

Check if rq_repmsg is NULL before retrieving
last committed transno from reply message.

Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: Ibf1e110e33df333934c65dfcf52870954e936180
Reviewed-on: http://review.whamcloud.com/17130
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>