sd: Avoid access to NULL BlockDriverState

Commit 4f8a066b (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: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/sd/sd.c

@@ -494,7 +494,7 @@ SDState *sd_init(BlockDriverState *bs, bool is_spi)
 {
     SDState *sd;
 
-    if (bdrv_is_read_only(bs)) {
+    if (bs && bdrv_is_read_only(bs)) {
         fprintf(stderr, "sd_init: Cannot use read-only drive\n");
         return NULL;
     }