Picture changes not proportionally after changing width to 0

Commit f74a961.

Steps to reproduce:
1. Open MuseScore, click on the first measure, add a Horizontal Frame.
2. Add a picture to it, check View->Inspector.
3. Click on your picture, change it's width to 0 with the Inspector.
4. Press "up" or enter a new value for the width.

Expected behavior: image height changes proportionally to it's width.
Actual behavior: image height is 99999.0 (seems like it is +Infinity clipped by maximum value of this QSpinBox).