Autodetect PASC mp1 files with 420 byte frame size (4 byte zero-padding)

[Carl Eugen Hoyos]

(videolan ticket 14538)
A vlc user uploaded a DCC PASC file which is a mp1 file from DCC that uses a 420 byte header containing four bytes zero-padding.
The file is not auto-detected by FFmpeg but can be converted if the mp3 format is forced. If the format was forced, the mp1 codec was correctly auto-detected until 5669ae84.

$ ffmpeg -f mp3 -acodec mp1 -i 2\ WIcky\ cut.MPP
ffmpeg version N-72127-g8252f63 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 23.101 / 54. 23.101
  libavcodec     56. 39.100 / 56. 39.100
  libavformat    56. 33.101 / 56. 33.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 16.101 /  5. 16.101
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
[mp1 @ 0x1bc2e60] Header missing
[mp3 @ 0x1bba4a0] Estimating duration from bitrate, this may be inaccurate
Input #0, mp3, from '2 WIcky cut.MPP':
  Duration: 00:00:53.33, start: 0.000000, bitrate: 384 kb/s
    Stream #0:0: Audio: mp1, 44100 Hz, stereo, s16p, 384 kb/s
At least one output file must be specified

Decoding the file with FFmpeg produces many warnings.

[Carl Eugen Hoyos]

The C source of a converter that contains a lengthy explanation in the header