The check was stupid (always was). This seems way more logical.

- This is REALLY slow.
- It is slow each time you Seek to somewhere that you have not been/played before.
- We ought to create a "Repair index" function instead, like for avi. (no time before release)

This close ticket #1687

Fix compilation warnings (here the lib might have ask for const char* instead of char as they don't modify the string at all)