RDMA/nes: Async event for closed QP causes crash

Under abnormal termination, modify_qp() closes the QP, and async event
(AE) handling also attempts to close the same QP, causing a crash.
Fix this by checking the state of the QP before processing the AE.

Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>