In places where an ignored return or a null-arg is a sure indication
of a bug add the GCC warning attributes. The null arg annotation
is not enable for Opus itself because it will cause the compiler
to optimize out some null checks. I don't trust our callers
quite that much.
Unlike the other headers, opus_multistream.h didn't declare
itself to be C code, causing confusing errors when the caller
tries to use the functions from C++.
