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

Closes: https://github.com/erikd/libsndfile/issues/92
psf_fseek (SF_PRIVATE *psf, sf_count_t offset, int whence)
{	sf_count_t	new_position ;

	if (psf->virtual_io)
		return psf->vio.seek (offset, whence, psf->vio_user_data) ;

	switch (whence)
	{	case SEEK_SET :
				offset += psf->fileoffset ;
				break ;

		case SEEK_END :
				if (psf->file.mode == SFM_WRITE)
				{	new_position = _lseeki64 (psf->file.filedes, offset, whence) ;

					if (new_position < 0)
						psf_log_syserr (psf, errno) ;

					return new_position - psf->fileoffset ;
					} ;

				/* Transform SEEK_END into a SEEK_SET, ie find the file
				** length add the requested offset (should be <= 0) to
				** get the offset wrt the start of file.
				*/
				whence = SEEK_SET ;
				offset = _lseeki64 (psf->file.filedes, 0, SEEK_END) + offset ;
				break ;

		default :
				/* No need to do anything about SEEK_CUR. */
				break ;
		} ;

	/*
	** Bypass weird Win32-ism if necessary.
	** _lseeki64() returns an "invalid parameter" error if called with the
	** offset == 0 and whence == SEEK_CUR.
	*** Use the _telli64() function instead.
	*/
	if (offset == 0 && whence == SEEK_CUR)
		new_position = _telli64 (psf->file.filedes) ;
	else
		new_position = _lseeki64 (psf->file.filedes, offset, whence) ;

	if (new_position < 0)
		psf_log_syserr (psf, errno) ;

	new_position -= psf->fileoffset ;

	return new_position ;
} /* psf_fseek */

psf_fseek (SF_PRIVATE *psf, sf_count_t offset, int whence)
{	sf_count_t	new_position ;

	if (psf->virtual_io)
		return psf->vio.seek (offset, whence, psf->vio_user_data) ;

	switch (whence)
	{	case SEEK_SET :
				offset += psf->fileoffset ;
				break ;

		case SEEK_END :
				if (psf->file.mode == SFM_WRITE)
				{	new_position = _lseeki64 (psf->file.filedes, offset, whence) ;

					if (new_position < 0)
						psf_log_syserr (psf, errno) ;

					return new_position - psf->fileoffset ;
					} ;

				/* Transform SEEK_END into a SEEK_SET, ie find the file
				** length add the requested offset (should be <= 0) to
				** get the offset wrt the start of file.
				*/
				whence = SEEK_SET ;
				offset = _lseeki64 (psf->file.filedes, 0, SEEK_END) + offset ;
				break ;

		default :
				/* No need to do anything about SEEK_CUR. */
				break ;
		} ;

	/*
	** Bypass weird Win32-ism if necessary.
	** _lseeki64() returns an "invalid parameter" error if called with the
	** offset == 0 and whence == SEEK_CUR.
	*** Use the _telli64() function instead.
	*/
	if (offset == 0 && whence == SEEK_CUR)
		new_position = _telli64 (psf->file.filedes) ;
	else
		new_position = _lseeki64 (psf->file.filedes, offset, whence) ;

	if (new_position < 0)
		psf_log_syserr (psf, errno) ;

	new_position -= psf->fileoffset ;

	return new_position ;
} /* psf_fseek */
