potential null pointer dereference in mux.c

[Shashank Sharma]

This is no longer an issue on the current master branch at the GitHub repository. Still filing it here, since this affects the version shipped with Ubuntu 22.04.

Due to an early return from init_ps (mux.c : 447), st->internal->priv_pts doesn't get initialized.
The return value of avformat_write_header is not checked and the ctxt is passed to
av_write_frame inside saveImage (file.c:148), inside av_write_frame, a null ptr deref can
happen when s->internal->pkt is referenced at mux.c:1214