Fix memory corruption in timidity resampling code.

From Sam Lantinga in SDL_mixer 1.2.8:
http://lists.libsdl.org/pipermail/commits-libsdl.org/2007-July/008673.html

Fixes a crash on OpenBSD.
Merge pull request #144 from jonathangray/timidity-resample-fix

Fix memory corruption in timidity resampling code