wav_io: guard against invalid channel numbers

Fixes #13
5 jobs for bugfix/speexenc-wav-header-channels in 2 minutes and 48 seconds (queued for 1 minute and 57 seconds)
Status Job ID Name Coverage
  Build
passed #10600
docker
autoconf

00:01:17

passed #10601
docker
fixed-point

00:00:49

passed #10604
docker
no-binaries

00:00:47

passed #10602
docker
no-float

00:00:49

passed #10603
docker
vorbis-psy

00:00:52