sd-bus: don't access invalid memory if a signal matcher was freed from its own callback

src/libsystemd/sd-bus/bus-match.c

@@ -293,6 +293,9 @@ int bus_match_run(
                         r = bus_maybe_reply_error(m, r, &error_buffer);
                         if (r != 0)
                                 return r;
+
+                        if (bus && bus->match_callbacks_modified)
+                                return 0;
                 }
 
                 return bus_match_run(bus, node->next, m);