renderonly: Fix potential NULL deref in the error path

scanout can be NULL.

Fixes: ad4d7ca833 ("kmsro: Fix renderonly_scanout BO aliasing")
Cc: mesa-stable
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23746>

src/gallium/auxiliary/renderonly/renderonly.c

@@ -114,7 +114,7 @@ renderonly_create_kms_dumb_buffer_for_resource(struct pipe_resource *rsc,
    return scanout;
 
 free_dumb:
-   destroy_dumb.handle = scanout->handle;
+   destroy_dumb.handle = create_dumb.handle;
    drmIoctl(ro->kms_fd, DRM_IOCTL_MODE_DESTROY_DUMB, &destroy_dumb);
 
    return NULL;