Fix audio glitch on restarts

[kallaballa]

At the moment on restart we hear a glitch on restart. That is because the audio buffer is still being played, but not filled. If we fill the buffer with silence the glitch will disappear

[kallaballa]

Filling with silence is the wrong approach. I found out that we can close the audio stream in two ways:

• call stopStream()
• or return non-zero from the render callback

so, if we detect a restart request we simply return non-zero and the stream will close cleanly without any glitches, because it seizes to use the audio buffer before there is an underun.

[kallaballa]

Fixed