moved source buffer struct member to local var in converter callback …

[AlexanderBollbach]

I have been reading the Core Audio book and referencing these public repositories. I noticed certain TODO's and took that as an opportunity to practice my git skills and open source etiquette. There are undoubtedly some problems with this commit and pull request but I would appreciate some recommended revisions and I would work on getting my push in order to be pulled. I realize this is a rather trivial contribution but simply a chance to practice working within github's ecosystem.