sipr: improve decoding of 16khz 16kbps mode

[ami_stuff]

this bug deserves it's own ticket

there are metalic artifacts between 21s-32s

C:\>ffmpeg -i sipr_16k_could_be_better.rm out.wav
ffmpeg version 3.2.git Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 4.6.2 (GCC)
  configuration: --disable-pthreads --enable-gpl
  libavutil      55. 43.100 / 55. 43.100
  libavcodec     57. 68.100 / 57. 68.100
  libavformat    57. 61.100 / 57. 61.100
  libavdevice    57.  2.100 / 57.  2.100
  libavfilter     6. 68.100 /  6. 68.100
  libswscale      4.  3.101 /  4.  3.101
  libswresample   2.  4.100 /  2.  4.100
  libpostproc    54.  2.100 / 54.  2.100
[rm @ 02243d60] Invalid stream index 1 for index at pos 81173
[sipr @ 0224d380] Warning: not compiled with thread support, using thread emulat
ion
Input #0, rm, from 'sipr_16k_could_be_better.
rm':
  Metadata:
    Generated By    : RealProducer 6.1.0.153 Windows
    Content Rating  : (PICS-1.1 "http://www.classify.org/safesurf" labels commen
t "RealProducer 6.1.0.153 Windows" ratings (SS~~000 1))
    Target Audiences: 28K Modem (20 Kbps);
    Audio Format    : Voice with Background Music
    Video Quality   : No Video
    Creation Date   : 12/19/2016 3:26:44
    Modification Date: 12/19/2016 3:26:44
    File ID         : 5001d8cc-b0c8-5861-36e0-07a19838af8b
    Keywords        :
    Abstract        :
    title           : Inp
    author          :
    copyright       : ę2016
    comment         :
  Duration: 00:00:38.31, start: 0.000000, bitrate: 16 kb/s
    Stream #0:0: Audio: sipr (sipr / 0x72706973), 16000 Hz, mono, flt, 16 kb/s
[sipr @ 025d9c00] Warning: not compiled with thread support, using thread emulat
ion
[pcm_s16le @ 02247140] Warning: not compiled with thread support, using thread e
mulation
Output #0, wav, to 'out.wav':
  Metadata:
    Generated By    : RealProducer 6.1.0.153 Windows
    Content Rating  : (PICS-1.1 "http://www.classify.org/safesurf" labels commen
t "RealProducer 6.1.0.153 Windows" ratings (SS~~000 1))
    Target Audiences: 28K Modem (20 Kbps);
    Audio Format    : Voice with Background Music
    Video Quality   : No Video
    Creation Date   : 12/19/2016 3:26:44
    Modification Date: 12/19/2016 3:26:44
    File ID         : 5001d8cc-b0c8-5861-36e0-07a19838af8b
    Keywords        :
    Abstract        :
    INAM            : Inp
    author          :
    ICOP            : ę2016
    ICMT            :
    ISFT            : Lavf57.61.100
    Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 16000 Hz, mono, s16,
256 kb/s
    Metadata:
      encoder         : Lavc57.68.100 pcm_s16le
Stream mapping:
  Stream #0:0 -> #0:0 (sipr (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
size=    1200kB time=00:00:38.31 bitrate= 256.6kbits/s speed= 189x
video:0kB audio:1200kB subtitle:0kB other streams:0kB global headers:0kB muxing
overhead: 0.008464%