sd-event: do not update signal fd after PID is changed

Otherwise, child event source will not work after the process is forked
and the event source is unref()ed on the child process.

src/libsystemd/sd-event/sd-event.c

@@ -706,6 +706,9 @@ static void event_unmask_signal_data(sd_event *e, struct signal_data *d, int sig
                 return;
         }
 
+        if (event_pid_changed(e))
+                return;
+
         assert(d->fd >= 0);
 
         if (signalfd(d->fd, &d->sigset, SFD_NONBLOCK|SFD_CLOEXEC) < 0)