pam: fix 16bpp grayscale mode

[ami_stuff]

attached file decodes correctly with netpbm

patch included

C:\>ffmpeg -i 16.pam
ffmpeg version N-36492-g7f83db3, Copyright (c) 2000-2012 the FFmpeg developers
  built on Jan  5 2012 01:57:12 with gcc 4.6.2
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-ru
ntime-cpudetect --enable-avisynth --enable-bzlib --enable-frei0r --enable-libope
ncore-amrnb --enable-libopencore-amrwb --enable-libfreetype --enable-libgsm --en
able-libmp3lame --enable-libopenjpeg --enable-librtmp --enable-libschroedinger -
-enable-libspeex --enable-libtheora --enable-libvo-aacenc --enable-libvo-amrwben
c --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-
libxvid --enable-zlib
  libavutil      51. 34.100 / 51. 34.100
  libavcodec     53. 53.100 / 53. 53.100
  libavformat    53. 29.100 / 53. 29.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 57.101 /  2. 57.101
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  5.100 /  0.  5.100
  libpostproc    51.  2.100 / 51.  2.100
Input #0, image2, from '16.pam':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: pam, gray, 1024x768, 25 tbr, 25 tbn, 25 tbc
At least one output file must be specified

[Carl Eugen Hoyos]

Did you test the patch?

[ami_stuff]

Yes, but only on big-endian. New patch attached.

[Carl Eugen Hoyos]

For future patches: Please always use "{ }" for if - else blocks.

Thank you for the patch!