sample-util: Fix "Darth Vader" panning bug
For muted channels, we forgot to increment a pointer, so if one
channel was muted but not the other, sound became distorted in a
Darth Vader like way. To test the difference, start two input
streams and pan one of them hard left (or right).
And hey, if you didn't think it sounded like Darth Vader, it's
your imagination that's broken, not mine! ;-)
BugLink: https://bugs.launchpad.net/bugs/928757
Signed-off-by: David Henningsson <david.henningsson@canonical.com>