Attempt to fix a compiler warning on Haiku.

(if this works...Haiku generates no warnings. I know, right?!?)
Explicitly write silence to the audio device while it is paused.

This is what SDL 1.2 did; we'll do this properly (add a method for the target
 driver to pause) when I rewrite all this code after the official 2.0 release.

Fixes Bugzilla #1857.
Whoops, missed a part of that last commit.

Actually fixes Bugzilla #1857.