IB/hfi1: Prevent a NULL dereference

In the original code, we set "fd->uctxt" to NULL and then dereference it
which will cause an Oops.

Fixes: f2a3bc00 ("IB/hfi1: Protect context array set/clear with spinlock")
Cc: <stable@vger.kernel.org> # 4.14.x
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>