flac : Fix for https://sourceforge.net/p/flac/bugs/425/

* flac/encode.c : Validate num_tracks field of cuesheet.
* libFLAC/stream_encoder.c : Add check for a NULL pointer.
* flac/encode.c : Improve bounds checking.

Closes: https://sourceforge.net/p/flac/bugs/425/
2 files changed