sd-event: Only register memory presure if write buffer size is zero

As documented in sd_event_add_memory_pressure(), we can only add
the memory pressure fd to epoll once we've written the watch string,
so make sure we don't register the memory pressure in
event_source_online() until we've written the watch string.

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

@@ -2987,9 +2987,11 @@ static int event_source_online(
                 break;
 
         case SOURCE_MEMORY_PRESSURE:
-                r = source_memory_pressure_register(s, enabled);
-                if (r < 0)
-                        return r;
+                if (s->memory_pressure.write_buffer_size == 0) {
+                        r = source_memory_pressure_register(s, enabled);
+                        if (r < 0)
+                                return r;
+                }
 
                 break;