return value of avcodec_decode_video2() not checked

[Shashank Sharma]

In src/film.cpp, the return value of avcodec_decode_video2() is not checked. The return value is negative in case of any error. This can lead to various faults down the line. Checked against shotdetect built from source on Ubuntu 22.04 (with patches applied).

ffmpeg docs for avcodec_decode_video2(): https://www.ffmpeg.org/doxygen/3.3/group__lavc__decoding.html#ga3ac51525b7ad8bca4ced9f3446e96532