[sudo-workers] Improve stack smashing protector check

[Todd C. Miller]

On Fri, 30 Oct 2015 15:56:12 -0000, Vicente Olivert Riera wrote:

> Indeed, now the problem is fixed. I'll backport the latest 3 commits
> until you release the new version. Any ETA for that to happen?

ETA is this coming Monday.

 - todd