hw/cxl: Check for zero length features in cmd_features_set_feature()

hw/cxl: Check for zero length features in cmd_features_set_feature()

Zero length data for features doesn't make any sense so exclude that case
early. This fixes the undefined behavior reported by coverity for a zero
length memcpy().

Resolves CID 1564900 and 1564901

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20241108175814.1248278-1-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>