fixes #2829 ofVbo custom attribute stride

addresses an issue where the buffer length would be calculated wrongly if stride for a custom ofVbo attribute was set to 0.

Based on @cinghiale's suggestion, and the ensueing discussion.

Signed-off-by: tgfrerer <tim@poniesandlight.co.uk>
Merge pull request #2839 from tgfrerer/fix-ofVboStride

fixes #2829 ofVbo custom attribute stride
Merge branch 'master' of https://github.com/openframeworks/openFrameworks.git into VS2013