ibcore: Allow passing NULL-pointers to ib_umem_release()

FreeBSD commit b633e08c705fe43180567eae26923d6f6f98c8d9 removed the
NULL-checks from the mlx4ib-driver.

This fixes the following NULL-pointer panic when unloading mlx4ib:
ib_umem_release()
mlx4_ib_destroy_qp()
ib_destroy_qp_user()
ipoib_transport_dev_cleanup()
ipoib_dev_cleanup()
ipoib_remove_one()
ib_unregister_client()
ipoib_cleanup_module()
linker_file_sysuninit()
linker_file_unload()
kern_kldunload()
amd64_syscall()

Linux commit:
836a0fbb3e76f704ad65ddfb57f00725245e509b

MFC after: 1 week
Submitted by: dandan@lysator.liu.se
Sponsored by: Lysator ACS
Sponsored by: NVIDIA Networking