Fixing failures when callbacks are unexpectedly undefined

I'm running into problems with trying to call `undefined` as a function; this doesn't handle unexpectedly-undefined results very gracefully. I'm fixing all the call-sites I could find in a cursory overview.

Forgive me if this is not following the project's stylistic requirements or anything … there was no documentation I could find, nor a channel on IRC or anything, describing the contribution guidelines for this project. (Nor, could I figure out how to re-compile the compilation products and browser-side include-able … so I just modified it by hand. Forgive me if I fucked that up.)