M4A export silently fails

[simonrob]

While #2 isn't currently reproducible, trying to export a narrative using the audio sample file from that issue fails silently.

[Icramss]

You can produce this bug if you can reproduce the issue #7

[simonrob]

ffprobe result from the file that does not work:

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'f3a058ee-05b1-4c10-9e21-ba1f59c74893.m4a':
  Metadata:
    major_brand     : dash
    minor_version   : 0
    compatible_brands: iso6mp41
    creation_time   : 2017-10-05T04:46:29.000000Z
  Duration: 00:06:07.01, start: 0.000000, bitrate: 127 kb/s
    Stream #0:0(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 3 kb/s (default)
    Metadata:
      creation_time   : 2017-10-05T04:46:29.000000Z
      handler_name    : SoundHandler

Interestingly, the length is listed as exactly half of that given by, e.g., QuickTime Player, which shows the file as twice as long, but only has audio present for the length shown in ffprobe.