How to handle `ftell()` failure?

[malespiaut]

Hello,

I wanted to contribute to PL_MPEG by adding safety checks around ftell() calls.

As mentioned in the standard C library manual, ftell() returns -1 in case of error.

Right now, PL_MPEG doesn't check the return value and treats a potential -1 as a valid offset.

I would have checked for the return value and called exit(EXIT_FAILURE) when ftell() returns -1, but I don't know if calling exit() goes against this project's design philosophy.

Best regards.