hnm: support 256x256 resolution

[ami_stuff]

there is one error message when I remove this check:

libavformat/hnm.c:

//   if (hnm->width  < 320 || hnm->width  > 640 ||
//       hnm->height < 150 || hnm->height > 480) {
//        av_log(s, AV_LOG_ERROR,
//               "invalid resolution: %ux%u\n", hnm->width, hnm->height);
//        return AVERROR_INVALIDDATA;
//   } 

​http://cd.textfiles.com/pcgamesexe/pcge199804/DREAMS/DATA/ANIM/DEMO.HNM

C:\>ffmpeg -i DEMO.HNM
ffmpeg version N-61338-g12ce58b Copyright (c) 2000-2014 the FFmpeg developers
  built on Mar 12 2014 23:35:57 with gcc 4.8.2 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-av
isynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enab
le-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetyp
e --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --ena
ble-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-l
ibopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libsp
eex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aa
cenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavp
ack --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable
-zlib
  libavutil      52. 66.101 / 52. 66.101
  libavcodec     55. 52.102 / 55. 52.102
  libavformat    55. 34.101 / 55. 34.101
  libavdevice    55. 11.100 / 55. 11.100
  libavfilter     4.  3.100 /  4.  3.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
[hnm @ 02cea760] invalid resolution: 256x256
DEMO.HNM: Invalid data found when processing input

[Michael Niedermayer]

Fixed in 79d82a65744b94406ac06677d42373879529926a