Don't initialize the audio buffer passed to the application's audio callback, since they are expected to entirely fill it with data or silence. For legacy apps that might expect the buffer to already have silence and thus may not fill the buffer in the callback, there's an environment variable to expose the old behaviour. Fixes Bugzilla #416.

The test programs in this directory tree are for demonstrating and
testing the functionality of the SDL library, and are placed in the
public domain.

October 28, 1997
--
	Sam Lantinga				(slouken@libsdl.org)