[OB] Add cleanup, source.c

[Joel Brobecker]

> >From gdbint.texinfo:
> 
>    Your function should explicitly do or discard the cleanups it
> creates.  Failing to do this leads to non-deterministic behavior since
> the caller will arbitrarily do or discard your functions cleanups.
> This need leads to two common cleanup styles.

Humpf! I think I have contributed loads of patches that do not
follow this advice. I'll be careful in the future, but sometimes
it's not easy. For instance, when you have a function that has
multiple possible exit points...

-- 
Joel