Insertion of silence was a bit broken since
df34b700 because the info whether or not the
source buffer supposed to be silence must be kept between callbacks. Failing to
do so causes rogue samples from the last buffer to be presented, I guess even a
crash can occur under some circumstances.

This patch uses a NULL audio_buf to keep the silence state across audio
callbacks.

Reviewed-by: Lukasz Marek <lukasz.m.luki2 at gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>