scsi-hd: fix property unset case

Commit c53659f0 ("BlockConf: Call backend functions to detect geometry
and blocksizes") causes a segmentation fault on the invalid
configuration of a scsi device without a drive.

Let's check for conf.blk before calling blkconf_blocksizes. The error
will be handled later on in scsi_realize anyway.
Reported-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NEkaterina Tumanova <tumanova@linux.vnet.ibm.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/scsi/scsi-disk.c

@@ -2291,7 +2291,12 @@ static void scsi_realize(SCSIDevice *dev, Error **errp)
 static void scsi_hd_realize(SCSIDevice *dev, Error **errp)
 {
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, dev);
-    blkconf_blocksizes(&s->qdev.conf);
+    /* can happen for devices without drive. The error message for missing
+     * backend will be issued in scsi_realize
+     */
+    if (s->qdev.conf.blk) {
+        blkconf_blocksizes(&s->qdev.conf);
+    }
     s->qdev.blocksize = s->qdev.conf.logical_block_size;
     s->qdev.type = TYPE_DISK;
     if (!s->product) {