Assertion fail when attempting to copy metadata

[Chippit]

Summary of the bug:
Using two input files, one with both an audio and video stream with some metadata, and a second with only an audio stream and no (or undefined) metadata, attempting to mux the video from file A with the audio from file B while copying metadata from the audio stream of file A results in an assertion fail.

How to reproduce:

Using any file A (temp.mkv), which can be generated as below (with some dummy language metadata for the audio stream) for testing purposes:

% ffmpeg -t 10 -s 200x200 -f rawvideo -pix_fmt rgb24 -r 25 -i /dev/zero -t 10 -ar 48000 -f s16le -acodec pcm_s16le -i /dev/zero -acodec copy -metadata:s:1 language=eng temp.mkv

And file B, 10 seconds of silence generated in Audacity and saved as m4a. Can also be generated with a similar command as above.

Attempt to run:

% ffmpeg -i temp.mkv -i temp.m4a -map 0:0 -map 1:0,0:1 -map_metadata:s:1 0:s:1 temp.mp4

Results in an error as shown below:

% ffmpeg -i temp.mkv -i temp.m4a -map 0:0 -map 1:0,0:1 -map_metadata:s:1 0:s:1 temp.mp4 -v 9 -loglevel 99
ffmpeg version N-43827-gc2271fa Copyright (c) 2000-2012 the FFmpeg developers
  built on Aug 23 2012 15:00:38 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-gpl --enable-libspeex --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-nonfree --enable-version3 --enable-x11grab
  libavutil      51. 70.100 / 51. 70.100
  libavcodec     54. 54.100 / 54. 54.100
  libavformat    54. 25.104 / 54. 25.104
  libavdevice    54.  2.100 / 54.  2.100
  libavfilter     3. 13.100 /  3. 13.100
  libswscale      2.  1.101 /  2.  1.101
  libswresample   0. 15.100 /  0. 15.100
  libpostproc    52.  0.100 / 52.  0.100
[matroska,webm @ 0xa429520] Format matroska,webm probed with size=2048 and score=100
st:0 removing common factor 1000000 from timebase
st:1 removing common factor 1000000 from timebase
[matroska,webm @ 0xa429520] File position before avformat_find_stream_info() is 683
[matroska,webm @ 0xa429520] parser not found for codec pcm_s16le, packets or times may be invalid.
[h264 @ 0xa430080] no picture 
[matroska,webm @ 0xa429520] parser not found for codec pcm_s16le, packets or times may be invalid.
[matroska,webm @ 0xa429520] All info found
[matroska,webm @ 0xa429520] File position after avformat_find_stream_info() is 3520
Guessed Channel Layout for  Input Stream #0.1 : mono
Input #0, matroska,webm, from 'temp.mkv':
  Metadata:
    ENCODER         : Lavf54.25.104
  Duration: 00:00:10.00, start: 0.000000, bitrate: 775 kb/s
    Stream #0:0, 3, 1/1000: Video: h264 (High 4:4:4 Predictive), yuv444p, 200x200, 1/50, SAR 1:1 DAR 1:1, 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
    Stream #0:1(eng), 1, 1/1000: Audio: pcm_s16le, 48000 Hz, mono, s16, 768 kb/s (default)
[mov,mp4,m4a,3gp,3g2,mj2 @ 0xa430d40] Format mov,mp4,m4a,3gp,3g2,mj2 probed with size=2048 and score=100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0xa430d40] ISO: File Type Major Brand: M4A 
[mov,mp4,m4a,3gp,3g2,mj2 @ 0xa430d40] File position before avformat_find_stream_info() is 112505
[mov,mp4,m4a,3gp,3g2,mj2 @ 0xa430d40] All info found
[mov,mp4,m4a,3gp,3g2,mj2 @ 0xa430d40] File position after avformat_find_stream_info() is 184
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'temp.m4a':
  Metadata:
    major_brand     : M4A 
    minor_version   : 512
    compatible_brands: isomiso2
    encoder         : Lavf53.21.0
  Duration: 00:00:10.00, start: 0.000000, bitrate: 89 kb/s
    Stream #1:0(und), 1, 1/44100: Audio: aac (mp4a / 0x6134706D), 44100 Hz, mono, s16, 86 kb/s
    Metadata:
      handler_name    : SoundHandler
File 'temp.mp4' already exists. Overwrite ? [y/N] y
Assertion 0 failed at ffmpeg_opt.c:449
Aborted (core dumped)