Fix issue with noise during startup

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>
git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12480 d7cf8633-e32d-0410-b094-e92efae38249