core: Don't abort when canceling a job through the private bus

Messages on the private bus have no sender. As it is now, when a job is
canceled through it, as "systemctl cancel" does when run as root, we
pass a NULL name to sd_bus_track_contains(), which fails an
assert_return(). In developer builds, that makes PID 1 abort. Otherwise
it's only logged at debug level, and we consult polkit, as we should.

Let's only check whether the sender owns the job for messages on the API
bus. That's the only bus bus_job_track_sender() tracks senders on, so
jobs enqueued through the private bus aren't tracked anyway.

(cherry picked from commit bfeac4c30e1a818cda14c8d1b347d500a9dfe4e3)

src/core/dbus-job.c

@@ -52,8 +52,10 @@ int bus_job_method_cancel(sd_bus_message *message, void *userdata, sd_bus_error
         if (r < 0)
                 return r;
 
-        /* Access is granted to the job owner */
-        if (!sd_bus_track_contains(j->bus_track, sd_bus_message_get_sender(message))) {
+        /* Access is granted to the job owner. Only clients on the API bus are tracked, messages on the
+         * private bus have no sender. */
+        if (sd_bus_message_get_bus(message) != j->manager->api_bus ||
+            !sd_bus_track_contains(j->bus_track, sd_bus_message_get_sender(message))) {
 
                 /* And for everybody else consult polkit */
                 r = bus_verify_manage_units_async(j->manager, message, reterr_error);