sd-bus: Fix a read after free error in bus-match. (#3624) (#3625)

The loop on bus_match_run should break and return immediately if
bus->match_callbacks_modified is true. Otherwise the loop may access
free'd data.

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

@@ -429,6 +429,9 @@ int bus_match_run(
                         r = bus_match_run(bus, c, m);
                         if (r != 0)
                                 return r;
+
+                        if (bus && bus->match_callbacks_modified)
+                                return 0;
                 }
         }