socket: free fdname member

Plug a small memory leak.

src/core/socket.c

@@ -165,6 +165,8 @@ static void socket_done(Unit *u) {
         free(s->user);
         free(s->group);
 
+        s->fdname = mfree(s->fdname);
+
         s->timer_event_source = sd_event_source_unref(s->timer_event_source);
 }