bus: move BUS_DONT_DESTROY calls after asserts

      It's not useful to bump the reference count before checking if the object is
NULL. Thanks to d40f5cc498 we can do this ;).

Related to https://bugzilla.redhat.com/show_bug.cgi?id=1576084,
https://bugzilla.redhat.com/show_bug.cgi?id=1575340,
https://bugzilla.redhat.com/show_bug.cgi?id=1575350. I'm not sure why those two
people hit this code path, while most people don't. At least we won't abort.