Return -1 in error condition instead of removing diff result.

BUG=skia:2160
R=epoger@google.com

Review URL: https://codereview.chromium.org/145123013

git-svn-id: http://skia.googlecode.com/svn/trunk@13407 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed