qcow2-bitmap: fix bitmap_free

Fix possible crash on error path in
qcow2_remove_persistent_dirty_bitmap. Although bitmap_free was added in
88ddffae8fc the bug was introduced later in commit 469c71edc72 (when
qcow2_remove_persistent_dirty_bitmap was added).

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170714123341.373857-1-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
