FTBFS with gcc-14

[umlaeute]

the last generation of gcc (GCC-14), has become considerably more strict when enforcing C-standards and pinter-correctness.

E.g. the following code

https://github.com/bplaum/gmerlin-avdecoder/blob/34de12ebd812712d2b146f329268362ccb944986/lib/bsf_adts.c#L147-L155

now fails to build from source, as av_packet_get_side_data wants a pointer to size_t rather than int.

The obvious fix is to declare:

 size_t extradata_size = 0; 

[bplaum]

Thanks for the notice. I got just warnings for using int instread of size_t (the int was from earlier ffmpegs). I converted all cases to size_t now.