wav_io: guard against invalid channel numbers

Fixes #13
5 jobs for master in 2 minutes and 32 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #10605
docker
autoconf

00:01:18

passed #10606
docker
fixed-point

00:00:50

passed #10609
docker
no-binaries

00:00:47

passed #10607
docker
no-float

00:00:50

passed #10608
docker
vorbis-psy

00:00:52