Fix the behaviour of FileGroup when seeking too far.

Previously, if you did a seek off the end of the file group,
the seek would return an error.

This is not what fseek() does; it returns no error, and preserves
the file pointer (returned by ftell()) as if the seek had been
successful.  fread()s after a too-far seek return no data, of
course.

Parsing some files (the example used to find the bug was a
H264 MP4) involves a seek which is to the byte after the end
of the mp4 file.  If this fails the whole header parsing fails
and DCP-o-matic refuses to use the file.