These ADPCM codecs include a per-frame flag that enables a raw 16-bit mode. Therefore
the the number of samples returned by get_nb_samples() is only ever approximate.

Fixes ticket #3460.
Signed-off-by: Peter Ross <pross@xvid.org>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>