librb: silence some fairly harmless compiler warnings

librb: silence some fairly harmless compiler warnings

These include warnings about "break" statements that will never be
executed (because they are after "return" statements), unused macros
(lost to code refactoring or never even used in the first place),
functions that call abort() or loop indefinitely but aren't marked with
the "noreturn" attribute, and use of variables possibly uninitialised
(a false positive).