buffer_size/4 is the value used by aplay. This fixes output to null
devices, e.g. writing ALSA output to a file.
(cherry picked from commit 8bfd7f6a475225a0595bf657f8b99a8fffb461e4)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Reinhard Tartler <siretart@tauware.de>