block: Assert that bdrv_release_dirty_bitmap succeeded

block: Assert that bdrv_release_dirty_bitmap succeeded

We use a loop over bs->dirty_bitmaps to make sure the caller is
only releasing a bitmap owned by bs. Let's also assert that in this case
the caller is releasing a bitmap that does exist.

Signed-off-by: Fam Zheng <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: John Snow <[email protected]>
Message-id: 1476395910[email protected]
Signed-off-by: Max Reitz <[email protected]>