Remove incorrect double-init of the z_stream for index compressing
Drop compression level for indexes to 5

Cuts the time to compress index files in half and increases the file
size by less than 5%.
Fix infinite loop on empty index files
Return the actual packet count from VisibleFrameCount() for audio tracks

The frame count for audio tracks is mostly meaningless as the API
doesn't expose audio frames, but it's the only way to check if a track
has been indexed.
Typo in FFMS_CreateIndexWithDemuxer definition
Merge pull request #150 from nixxquality/master

(Documentation) Typo in FFMS_CreateIndexWithDemuxer definition