srtdec fails to probe files whose first event ID is not 1 or 0

[Ridley Combs]

Summary of the bug:
Some SRT files in the wild don't start with 0 or 1. These files are currently not probed by srtdec, because it expects the first item in the file to atoi() to 0 or 1. Amusingly enough, the probe returns AVPROBE_SCORE_MAX if there's no event ID at all, but the first event starts at a time <2 hours.
How to reproduce:

$ ffmpeg -i test.srt -f srt -
ffmpeg version N-66154-g1654ca7 Copyright (c) 2000-2014 the FFmpeg developers
  built on Sep  5 2014 23:23:09 with Apple LLVM version 6.0 (clang-600.0.34.4) (based on LLVM 3.5svn)
  configuration: --extra-cflags='-march=native' --enable-optimizations --disable-stripping --enable-debug --enable-libx265 --enable-gpl --enable-libopus --enable-version3 --enable-nonfree --enable-postproc --enable-libass --enable-libfdk-aac --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-gnutls --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxvid --prefix=/usr/local --enable-static --enable-shared --enable-avresample --enable-vda --enable-opencl --enable-libzvbi --cc=clang
  libavutil      54.  7.100 / 54.  7.100
  libavcodec     56.  1.100 / 56.  1.100
  libavformat    56.  4.100 / 56.  4.100
  libavdevice    56.  0.100 / 56.  0.100
  libavfilter     5.  1.100 /  5.  1.100
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  0.100 /  3.  0.100
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  0.100 / 53.  0.100
test.srt: Invalid data found when processing input

Example file content:

3
00:00:00,000 --> 00:00:05,000
TEST1

4
00:00:05,100 --> 00:00:05,250
TEST2

5
00:00:05,500 --> 00:08:47,307
TEST3

[Clément Bœsch]

Fixed in 9ae83ece8a8e92d7e9b7a077eb10d324991f0453