improve iac decoder

[ami_stuff]

currently ffmpeg's decoder is not as good as the original one

​https://samples.ffmpeg.org/A-codecs/Indeo_audio/hvalen.avi

for example there are some overflows or something with this file:

​https://samples.ffmpeg.org/A-codecs/Indeo_audio/2heelsidemobe.AVI

C:\>ffmpeg -i C:\2heelsidemobe.AVI out.avi
ffmpeg version N-78758-g5156578 Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.3.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --
enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-l
ibilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enab
le-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --en
able-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --ena
ble-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --
enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-lib
x265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-d
ecklink --enable-zlib
  libavutil      55. 19.100 / 55. 19.100
  libavcodec     57. 27.100 / 57. 27.100
  libavformat    57. 26.100 / 57. 26.100
  libavdevice    57.  0.101 / 57.  0.101
  libavfilter     6. 37.100 /  6. 37.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.101 /  2.  0.101
  libpostproc    54.  0.100 / 54.  0.100
[avi @ 038de6e0] non-interleaved AVI
Input #0, avi, from 'C:\2heelsidemobe.AVI':
  Duration: 00:00:02.87, start: 0.000000, bitrate: 1255 kb/s
    Stream #0:0: Video: indeo4 (IV41 / 0x31345649), yuv410p, 240x180, 1099 kb/s,
 15 fps, 15 tbr, 15 tbn, 15 tbc
    Metadata:
      title           : C:\Galleries\Main Gallery\TMPVIDEO\IVCSATMP.AVI
    Stream #0:1: Audio: iac ([2][4][0][0] / 0x0402), 44100 Hz, stereo, fltp, 176
 kb/s
    Metadata:
      title           : C:\Galleries\Main Gallery\TMPVIDEO\IVCSATMP.AVI
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf57.26.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 240x180, q=2-31, 200
 kb/s, 15 fps, 15 tbn, 15 tbc
    Metadata:
      title           : C:\Galleries\Main Gallery\TMPVIDEO\IVCSATMP.AVI
      encoder         : Lavc57.27.100 mpeg4
    Side data:
      cpb: bitrate max/min/avg: 0/0/200000 buffer size: 0 vbv_delay: -1
    Stream #0:1: Audio: mp3 (libmp3lame) (U[0][0][0] / 0x0055), 44100 Hz, stereo
, fltp
    Metadata:
      title           : C:\Galleries\Main Gallery\TMPVIDEO\IVCSATMP.AVI
      encoder         : Lavc57.27.100 libmp3lame
Stream mapping:
  Stream #0:0 -> #0:0 (indeo4 (native) -> mpeg4 (native))
  Stream #0:1 -> #0:1 (iac (native) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
[iac @ 038e06a0] Potential problem on band 26, coefficient 187: cw_len=1
[iac @ 038e06a0] Potential problem on band 26, coefficient 188: cw_len=1
[iac @ 038e06a0] Potential problem on band 26, coefficient 189: cw_len=1
[iac @ 038e06a0] Potential problem on band 26, coefficient 190: cw_len=1
[iac @ 038e06a0] Potential problem on band 26, coefficient 192: cw_len=1
[iac @ 038e06a0] Potential problem on band 26, coefficient 193: cw_len=1
[iac @ 038e06a0] Potential problem on band 26, coefficient 194: cw_len=1
frame=   41 fps=0.0 q=10.5 Lsize=     187kB time=00:00:02.86 bitrate= 534.9kbits
/s speed=10.8x
video:129kB audio:44kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 7.720324%

[ami_stuff]

see also #3886