IB/mlx5: Fix NULL pointer dereference on debug print

IB/mlx5: Fix NULL pointer dereference on debug print

commit a1ab8402d15d2305d2315d96ec3294bfdf16587e upstream.

For XRC QP CQs may not exist. Check before attempting dereference.

Fixes: e126ba97dba9 ('mlx5: Add driver for Mellanox Connect-IB adapters')
Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>