sdlaudio: always clear the sample buffer

sdlaudio: always clear the sample buffer

Always fill the remaining audio callback buffer with silence.
SDL 2.0 doesn't initialize the audio callback buffer. This was
an incompatible change compared to SDL 1.2. For reference read
the SDL 1.2 to 2.0 migration guide.

Signed-off-by: Volker Rümelin <[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Message-id: 9315afe5-5958-c0b4-ea1e-14769511a9d5@t-online.de
Message-Id: <20210110100239[email protected]>
Signed-off-by: Gerd Hoffmann <[email protected]>