drm/virtio: Ensure that objs is not NULL in virtio_gpu_array_put_free()

If virtio_gpu_object_shmem_init() fails (e.g. due to fault injection, as it
happened in the bug report by syzbot), virtio_gpu_array_put_free() could be
called with objs equal to NULL.

Ensure that objs is not NULL in virtio_gpu_array_put_free(), or otherwise
return from the function.

Cc: stable@vger.kernel.org # 5.13.x
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reported-by: syzbot+e9072e90624a31dfa85f@syzkaller.appspotmail.com
Fixes: 377f8331d0 ("drm/virtio: fix possible leak/unlock virtio_gpu_object_array")
Link: http://patchwork.freedesktop.org/patch/msgid/20211213183122.838119-1-roberto.sassu@huawei.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/virtio/virtgpu_gem.c

@@ -248,6 +248,9 @@ void virtio_gpu_array_put_free(struct virtio_gpu_object_array *objs)
 {
 	u32 i;
 
+	if (!objs)
+		return;
+
 	for (i = 0; i < objs->nents; i++)
 		drm_gem_object_put(objs->objs[i]);
 	virtio_gpu_array_free(objs);