[merge][#330248] don't crash on incorrect progress bar updates

[Bundle Buggy]

Bundle Buggy has detected this merge request.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3Ce01316480903021822m6b183094gb8c8ba3b216a674e%40mail.gmail.com%3E
Project: Bazaar