src/file_io.c : Prevent potential divide-by-zero.

Closes: https://github.com/erikd/libsndfile/issues/92
psf_fsync (SF_PRIVATE *psf)
{	FlushFileBuffers (psf->file.handle) ;
} /* psf_fsync */

psf_fsync (SF_PRIVATE *psf)
{	FlushFileBuffers (psf->file.handle) ;
} /* psf_fsync */
