benchmark only works if encoding is possible

[Carl Eugen Hoyos]

The -benchmark option cannot be used to test the memory (and process time) requirement to probe the input file:

$ ffmpeg -benchmark -i tests/lena.pnm
ffmpeg version N-50189-g239b88c Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb 22 2013 23:46:57 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl --disable-indev=jack
  libavutil      52. 17.102 / 52. 17.102
  libavcodec     54. 92.100 / 54. 92.100
  libavformat    54. 63.100 / 54. 63.100
  libavdevice    54.  3.103 / 54.  3.103
  libavfilter     3. 39.101 /  3. 39.101
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, image2, from 'tests/lena.pnm':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: ppm, rgb24, 256x256, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

$ ffmpeg -benchmark -i tests/lena.pnm -f null -
ffmpeg version N-50189-g239b88c Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb 22 2013 23:46:57 with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl --disable-indev=jack
  libavutil      52. 17.102 / 52. 17.102
  libavcodec     54. 92.100 / 54. 92.100
  libavformat    54. 63.100 / 54. 63.100
  libavdevice    54.  3.103 / 54.  3.103
  libavfilter     3. 39.101 /  3. 39.101
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, image2, from 'tests/lena.pnm':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: ppm, rgb24, 256x256, 25 tbr, 25 tbn, 25 tbc
Output #0, null, to 'pipe:':
  Metadata:
    encoder         : Lavf54.63.100
    Stream #0:0: Video: rawvideo (RGB[24] / 0x18424752), rgb24, 256x256, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (ppm -> rawvideo)
Press [q] to stop, [?] for help
[null @ 0x2836ca0] Encoder did not produce proper pts, making some up.
frame=    1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A
video:0kB audio:0kB subtitle:0 global headers:0kB muxing overhead -122.916667%
bench: utime=0.000s maxrss=4996kB