g721: support stereo modes

[ami_stuff]

decodes ok with winamp

decodes not so good with sox (probably buggy support)

​https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket1955/g721_2ch_44khz.au

C:\>ffmpeg -i C:\g721_2ch_44khz.au out.wav
ffmpeg version N-79632-g3ce1988 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.5.0 (GCC) 20100414 (Fedora MinGW 4.5.0-1.fc14)
  configuration: --prefix=/var/www/users/research/ffmpeg/snapshots/build --arch=
x86 --target-os=mingw32 --cross-prefix=i686-pc-mingw32- --cc='ccache i686-pc-min
gw32-gcc' --enable-pthreads --enable-memalign-hack --enable-runtime-cpudetect --
enable-cross-compile --enable-static --disable-shared --extra-libs='-lws2_32 -lw
inmm -lpthread' --extra-cflags='--static -I/var/www/users/research/ffmpeg/snapsh
ots/build/include' --extra-ldflags='-static -L/var/www/users/research/ffmpeg/sna
pshots/build/lib' --enable-bzlib --enable-zlib --enable-gpl --enable-version3 --
enable-nonfree --enable-libx264 --enable-libspeex --enable-libtheora --enable-li
bvorbis --enable-libfaac --enable-libxvid --enable-libopencore-amrnb --enable-li
bopencore-amrwb --enable-libmp3lame --enable-libfreetype --enable-libvpx --disab
le-decoder=libvpx
  libavutil      55. 22.101 / 55. 22.101
  libavcodec     57. 38.100 / 57. 38.100
  libavformat    57. 34.103 / 57. 34.103
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 44.100 /  6. 44.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[g726le @ 0x28ad4a0] Decoding more than one channel is not implemented. Update y
our FFmpeg version to the newest one from Git. If the problem still occurs, it m
eans that your file has a feature which has not been implemented.
[g726le @ 0x28ad4a0] If you want to help, upload a sample of this file to ftp://
upload.ffmpeg.org/incoming/ and contact the ffmpeg-devel mailing list. (ffmpeg-d
evel@ffmpeg.org)
[au @ 0x28a4020] Failed to open codec in av_find_stream_info
[g726le @ 0x28ad4a0] Decoding more than one channel is not implemented. Update y
our FFmpeg version to the newest one from Git. If the problem still occurs, it m
eans that your file has a feature which has not been implemented.
[g726le @ 0x28ad4a0] If you want to help, upload a sample of this file to ftp://
upload.ffmpeg.org/incoming/ and contact the ffmpeg-devel mailing list. (ffmpeg-d
evel@ffmpeg.org)
Guessed Channel Layout for  Input Stream #0.0 : stereo
Input #0, au, from 'C:\g721_2ch_44khz.au':
  Duration: 00:00:30.01, start: 0.000000, bitrate: 352 kb/s
    Stream #0:0: Audio: adpcm_g726le ([23][0][0][0] / 0x0017), 44100 Hz, 2 chann
els, 352 kb/s
File 'out.wav' already exists. Overwrite ? [y/N] y
[abuffer @ 0x2c49700] Unable to parse option value "(null)" as sample format
    Last message repeated 1 times
[abuffer @ 0x2c49700] Error setting option sample_fmt to value (null).
[graph 0 input from stream 0:0 @ 0x2c49640] Error applying options to the filter
.
Error opening filters!