Don't send silence if buffer is underrun in infinte loop

I don't understand why it should send silence if the buffer is underrun. Not only does it garble the output, if you pipe to a file this file grows several into GB sizes in seconds? That can't be good.

It also lowers CPU usage in my tests. 

reproduce:
touch /tmp/output.bin
./shairport -o pipe /tmp/output.bin

Connect Airplay device, stream for a bit. Shut it down, and look at the size of /tmp/output.bin