The usage of restrict in include/opus_custom.h was missed
by the prior commit replacing this keyword with a macro.
Also fixes some prototype/function agreement with respect to
restrict.
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.
