block: Fix anonymous BBs in blk_root_inactivate()

blk->name isn't an array, but a pointer that can be NULL. Checking for
an anonymous BB must involve a NULL check first, otherwise we get
crashes.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>

block/block-backend.c

@@ -168,7 +168,7 @@ static int blk_root_inactivate(BdrvChild *child)
      * this point because the VM is stopped) and unattached monitor-owned
      * BlockBackends. If there is still any other user like a block job, then
      * we simply can't inactivate the image. */
-    if (!blk->dev && !blk->name[0]) {
+    if (!blk->dev && !blk_name(blk)[0]) {
         return -EPERM;
     }