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

In the Linux kernel, the following vulnerability has been resolved: 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.