nvme-fc: Prevent null pointer dereference in nvme_fc_io_getuuid()

The nvme_fc_fcp_op structure describing an AEN operation is initialized with a
null request structure pointer. An FC LLDD may make a call to
nvme_fc_io_getuuid passing a pointer to an nvmefc_fcp_req for an AEN operation.

Add validation of the request structure pointer before dereference.

Signed-off-by: Nigel Kirkland <nkirkland2304@gmail.com>
Reviewed-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>