av: Cast AVContext::bit_rate to a guint before passing to varargs functions

av: Cast AVContext::bit_rate to a guint before passing to varargs functions

We expect it to be a int or uint, however it changed the type to a
int64_t in later versions of ffmpeg. As such it would be passed as a 64
bit value to varargs functions, while the consumer of the arguments
assumes only 32 bits. This causes crashes.

https://bugzilla.gnome.org/show_bug.cgi?id=771092