src/flac.c : Fix seeking to offset 0 from end of file.

For FLAC files, (unlike WAV and others), seeking to the end of the
file using:

    file_length = sf_seek (file, 0, SEEK_END) ;

was failing because the underlying libFLAC was considering this as
an error condition. Work around this by detecting this in libsndfile
and doing the right thing.

Closes : https://github.com/erikd/libsndfile/issues/34
Add a couple of 'seek to end of file' tests.