alsaloop: Fix the silence generation in xrun_sync

alsaloop: Fix the silence generation in xrun_sync

The silence frames generation in xrun_sync() seems to be currently
a bit off as snd_pcm_format_set_silence() uses a wrong buffer address
and size for zero-ing out the data. Consequently instead of clearing
out the last frame(s), snd_pcm_format_set_silence() ends up silencing
samples somewhere else in the buffer. This is partilarly more obvious
for higher frame sizes (e.g. more than 2 channels).

This patch fixes this issue by correcting the parameters passed to
snd_pcm_format_set_silence().

BugLink: https://github.com/alsa-project/alsa-utils/pull/58
Signed-off-by: Dragos Tarcatu <dragos_tarcatu@mentor.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>