Do not allow writing invalid wav channel layouts by default.

Neither WMP nor QT play wav files with too large channel layouts.
Fixes ticket #3543.
Remove an unneeded include of avassert.h from amr.c.
Fix a typo in amr.c.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Fix a typo in amr.c.
  Remove an unneeded include of avassert.h from amr.c.
  Do not allow writing invalid wav channel layouts by default.

Merged-by: Michael Niedermayer <michaelni@gmx.at>