block: Null pointer dereference in blk_root_get_parent_desc()

The dev_id returned by the call to blk_get_attached_dev_id() in
blk_root_get_parent_desc() can be NULL (an internal call to
object_get_canonical_path may have returned NULL).

Instead of just checking this case before before dereferencing,
adjust blk_get_attached_dev_id() to return the empty string if no
object path can be found (similar to the case when blk->dev is NULL
and an empty string is returned).
Signed-off-by: NLiam Merwick <Liam.Merwick@oracle.com>
Message-id: 1541453919-25973-3-git-send-email-Liam.Merwick@oracle.com
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block/block-backend.c

@@ -918,7 +918,8 @@ char *blk_get_attached_dev_id(BlockBackend *blk)
     } else if (dev->id) {
         return g_strdup(dev->id);
     }
-    return object_get_canonical_path(OBJECT(dev));
+
+    return object_get_canonical_path(OBJECT(dev)) ?: g_strdup("");
 }
 
 /*