block-backend: avoid bdrv_unregister_buf() NULL pointer deref

block-backend: avoid bdrv_unregister_buf() NULL pointer deref

bdrv_*() APIs expect a valid BlockDriverState. Calling them with bs=NULL
leads to undefined behavior.

Jonathan Cameron reported this following NULL pointer dereference when a
VM with a virtio-blk device and a memory-backend-file object is
terminated:
1. qemu_cleanup() closes all drives, setting blk->root to NULL
2. qemu_cleanup() calls user_creatable_cleanup(), which results in a RAM
   block notifier callback because the memory-backend-file is destroyed.
3. blk_unregister_buf() is called by virtio-blk's BlockRamRegistrar
   notifier callback and undefined behavior occurs.

Fixes: baf422684d73 ("virtio-blk: use BDRV_REQ_REGISTERED_BUF optimization hint")
Co-authored-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20221121211923.1993171-1-stefanha@redhat.com>