id3tag: do not add video stream when size of APIC is 0

[ami_stuff]

It looks like some ID3Tag editros saves APIC tag even when no real pic is saved to ID3,
so as a result FFplay can't play the file (audio part), because it fails at the swscale's conversion.

I will send a possible patch to ffmpeg-devel soon which will reject APICs with size of 0.

C:\>ffplay split.mp3
ffplay version N-40593-g6d37634 Copyright (c) 2003-2012 the FFmpeg developers
  built on May 11 2012 12:12:53 with gcc 4.6.3
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libass
 --enable-libcelt --enable-libopencore-amrnb --enable-libopencore-amrwb --enable
-libfreetype --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libope
njpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libth
eora --enable-libutvideo --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-
libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --e
nable-zlib
  libavutil      51. 50.100 / 51. 50.100
  libavcodec     54. 21.101 / 54. 21.101
  libavformat    54.  4.100 / 54.  4.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 72.105 /  2. 72.105
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 11.100 /  0. 11.100
  libpostproc    52.  0.100 / 52.  0.100
[mp3 @ 0224d7e0] max_analyze_duration 5000000 reached at 5015510
[mp3 @ 0224d7e0] decoding for stream 1 failed
[mp3 @ 0224d7e0] Could not find codec parameters (Video: mjpeg)
[mp3 @ 0224d7e0] Estimating duration from bitrate, this may be inaccurate
Input #0, mp3, from 'split.mp3':
  Metadata:
[cut]
  Duration: 00:00:09.36, start: 0.000000, bitrate: 223 kb/s
    Stream #0:0: Audio: mp3, 44100 Hz, stereo, s16, 224 kb/s
    Stream #0:1: Video: mjpeg, 90k tbr, 90k tbn, 90k tbc
    Metadata:
      title           :
      comment         : Other
[format @ 02256d60] auto-inserting filter 'auto-inserted scale 0' between the fi
lter 'src' and the filter 'format'
Impossible to convert between the formats supported by the filter 'src' and the
filter 'auto-inserted scale 0'
   3.36 A-V:-1337451918.484 fd=   0 aq=  155KB vq=    0KB sq=    0B f=0/0

[Carl Eugen Hoyos]

Fixed by you.