core: Pass NULL error in dump_impl()

core: Pass NULL error in dump_impl()

If mac_selinux_access_check() or bus_verify_bypass_dump_ratelimit_async()
fail, we goto "ratelimited" where we set a custom D-BUS error. In
"ratelimited", we call sd_bus_error_setf() which eventually hits an
assert_return(!bus_error_is_dirty()). Avoid hitting this assertion by
passing NULL as the error to mac_selinux_access_check() and
bus_verify_bypass_dump_ratelimit_async() since we will override the error
immediately anyway if either fails.

We modify both functions as well to allow passing a NULL error and fix
the argument name as well while we're at it.