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>

hw/cxl/cxl-mailbox-utils.c

@@ -1288,6 +1288,10 @@ static CXLRetCode cmd_features_set_feature(const struct cxl_cmd *cmd,
     set_feat_info->data_offset = hdr->offset;
     bytes_to_copy = len_in - sizeof(CXLSetFeatureInHeader);
 
+    if (bytes_to_copy == 0) {
+        return CXL_MBOX_INVALID_PAYLOAD_LENGTH;
+    }
+
     if (qemu_uuid_is_equal(&hdr->uuid, &patrol_scrub_uuid)) {
         if (hdr->version != CXL_MEMDEV_PS_SET_FEATURE_VERSION) {
             return CXL_MBOX_UNSUPPORTED;