RDMA/siw: Properly check send and receive CQ pointers

The check for the NULL of pointer received from container_of() is
incorrect by definition as it points to some offset from NULL.

Change such check with proper NULL check of SIW QP attributes.

Fixes: 303ae1cd ("rdma/siw: application interface")
Link: https://lore.kernel.org/r/a7535a82925f6f4c1f062abaa294f3ae6e54bdd2.1620560310.git.leonro@nvidia.com


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>