onscreen/native: Fix fd error check

meta_drm_buffer_dumb_ensure_dmabuf_fd () return -1 on error, not 0.

Fix the error check.

Fixes: 84bde805fe34 ("native: Consolidate DRM buffer management to MetaDrmBuffer types")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4109>

src/backends/native/meta-onscreen-native.c

@@ -1028,7 +1028,7 @@ copy_shared_framebuffer_primary_gpu (CoglOnscreen                        *onscre
   g_assert (format_info);
 
   dmabuf_fd = meta_drm_buffer_dumb_ensure_dmabuf_fd (buffer_dumb, &error);
-  if (!dmabuf_fd)
+  if (dmabuf_fd < 0)
     {
       meta_topic (META_DEBUG_KMS,
                   "Failed to create DMA buffer: %s", error->message);