CRAS: mix - stop mixing if shm is out of samples.
CRAS: mix - stop mixing if shm is out of samples.

In case the shm region gets modified and there are no longer enough
samples to read, exit.  Before this, the code would loop forever
waiting for more samples, that may never arrive.

BUG=chromium:255711
TEST=play audio from multiple streams

Change-Id: I204885d20ce2a7374fd7f228e451015c94481641
Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/60566
Reviewed-by: Chih-Chung Chang <ebe32a3bdce230d12f75781016b6b880113fb486@chromium.org>

drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd,drinkcat/adhd