[libdvdread] - fix out of bound access when playing some iso files

ATM playing iso files on ios leads to instant crash. The reason is a bad access because of a typo.

thx to @Karlson2k for finding that one out.

This PR fixes the crash - playback of iso on ios is still broken though - there is still something wrong with the cached reads after e502b0993760b3e8b001f0360755a424bab79086

This is a regression from Gotham and we can hopefully track it down before release...