blockdev: Add missing bdrv_unref() in drive-backup

All error paths after a successful bdrv_open() of target_bs should
contain a bdrv_unref(target_bs). This one did not yet, so add it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/blockdev.c b/blockdev.c
index 917ae06..07c1741 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3171,6 +3171,7 @@
         bmap = bdrv_find_dirty_bitmap(bs, bitmap);
         if (!bmap) {
             error_setg(errp, "Bitmap '%s' could not be found", bitmap);
+            bdrv_unref(target_bs);
             goto out;
         }
     }