sd-bus: mark sd_bus_unref() as broken regarding self-refs

If you allocate a message with bus==NULL and then unref the main bus,
it will free your message underneath and your program will go boom!

To fix that, we really need to figure out what the semantics for
self-references (m->bus) should be and when/where/what accesses are
actually allowed.

Same is true for the pseudo-thread-safety we employ..