drm/syncobj: Fix oops on drm_syncobj_find_fence(file_priv, 0, ...).

This broke rendering on V3D, where we almost always have a 0
in-syncobj.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 48197bc5 ("drm: add syncobj timeline support v9")
Cc: Chunming Zhou <david1.zhou@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.kernel.org/patch/10669317/Signed-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/drm_syncobj.c

@@ -443,7 +443,8 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
 	int ret;
 
 	ret = drm_syncobj_search_fence(syncobj, point, flags, fence);
-	drm_syncobj_put(syncobj);
+	if (syncobj)
+		drm_syncobj_put(syncobj);
 	return ret;
 }
 EXPORT_SYMBOL(drm_syncobj_find_fence);