Fix issue #36

git-svn-id: http://vapoursynth.googlecode.com/svn/trunk@440 f9120d27-2007-6f97-8312-0f4ebfa7710f
Throw an exception on out of range frame requests, fixes issue #37

git-svn-id: http://vapoursynth.googlecode.com/svn/trunk@441 f9120d27-2007-6f97-8312-0f4ebfa7710f