RDMA/i40iw: fix a potential NULL pointer dereference

commit 04db1580 upstream.

A NULL pointer can be returned by in_dev_get(). Thus add a corresponding
check so that a NULL pointer dereference will be avoided at this place.

Fixes: 8e06af71

 ("i40iw: add main, hdr, status")
Link: https://lore.kernel.org/r/1577672668-46499-1-git-send-email-xiyuyang19@fudan.edu.cn
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>