LU-18897 o2iblnd: NULL pointer dereference

LU-18897 o2iblnd: NULL pointer dereference

When the network is flapping, we could get an
RDMA_CM_EVENT_UNREACHABLE event before conn is created, so we should
check the value first.

Test-Parameters: trivial
Signed-off-by: Cyril Bordage <cbordage@whamcloud.com>
Change-Id: I8d9777370b927c28ee438687de596e498d64bb07
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59498
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>