sd: Avoid access to NULL BlockDriverState

Commit 4f8a066b5fc254eeaabbbde56ba4f5b29cc68fdf (blockdev: Remove IF_*
check for read-only blockdev_init) added a usage of bdrv_is_read_only()
to sd_init(), which is called for versatilepb, versatileab and
xilinx-zynq-a9 machines among others with NULL argument by default,
causing the new qom-test to fail.

Add a check to prevent this.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
