Change mixer position state to unsigned

In rare cases the position variable overflows if it's a signed integer,
so change it to unsigned. Happens in bluish.xm.

Anonymous report via Android crash feedback:

3.9.3
Nexus 5 (hammerhead)
xmp player crashes after playing several seconds of Probe - Bluish track
(The Black Lotus team) thank you.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update changelog

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>