Fix issue with noise during startup
After migrating to SDL2 interfaces there is noise during startup fix this by initializng buffer as specification requires.
Per SDL_AudioCallback spec: The callback must completely initialize the buffer; as of SDL 2.0, this buffer is not initialized before the callback is called. If there is nothing to play, the callback should fill the buffer with silence.
Signed-off-by: Amadeusz Sławiński amade@asmblr.net