socket: fix segfault

TODO

@@ -43,6 +43,8 @@ CGroup Rework Completion:
 
 Features:
 
+* sd-bus: synthesized messages should get serial number (uint32_t) -1
+
 * sd-event: allow multiple signal handlers per signal
 
 * timer: expose accuracy as unit setting

src/core/load-fragment.c

@@ -298,6 +298,7 @@ int config_parse_socket_listen(const char *unit,
         }
 
         p->fd = -1;
+        p->socket = s;
 
         if (s->ports) {
                 LIST_FIND_TAIL(port, s->ports, tail);