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 <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1476395910-8697-7-git-send-email-jsnow@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
