[Qemu-block] [PATCH] migration: Appease coverity, skip empty block trees

[John Snow]

If a tree consists exclusively of implicit filter nodes, we might crash
QEMU. This configuration should not exist in practice, but if it did,
skipping it would be fine.

For the purposes of debug builds, throw an assert to remind us that
this configuration is truly unexpected, but if it's compiled out we
will cope just fine.

Signed-off-by: John Snow <address@hidden>
---
 migration/block-dirty-bitmap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index 3bafbbdc4c..02725293dd 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -287,6 +287,10 @@ static int init_dirty_bitmap_migration(void)
         while (bs && bs->drv && bs->implicit) {
             bs = backing_bs(bs);
         }
+        if (!bs) {
+            g_assert_not_reached();
+            continue;
+        }
 
         for (bitmap = bdrv_dirty_bitmap_next(bs, NULL); bitmap;
              bitmap = bdrv_dirty_bitmap_next(bs, bitmap))
-- 
2.14.4