Applied patch from Brett Trotter that stuffs zeros into the head of

Applied patch from Brett Trotter that stuffs zeros into the head of
the tx signal processing pipeline when the Tx FIFO is empty.

This results in the DACs outputing zeros when there's no data, unless
the tx pipeline is disabled on the host.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@4287 221aa14e-8319-0410-a670-987f0aec2ac5