Fix a couple of minor bugs with build/sanitize-png-files.sh

Fix a couple of minor bugs with build/sanitize-png-files.sh

The bugs fixed are:
- programs required for -o2 weren't checked for when using -o2
but were checked for lower -o levels.
- if no savings could be found the results printed at the end
were confusing and the script had a divide by zero error.

BUG=None

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=203051

[oshima, 2013-05-29 14:35:53]

lgtm

thank you for fixing them!

[commit-bot: I haz the power, 2013-05-30 01:32:53]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/benwells@chromium.org/15970004/1

[commit-bot: I haz the power, 2013-05-30 03:56:56]

Change committed as 203051