sd-bus: when running user find function don't trust the value to be initialized

The find function is externally provided, and we shouldn't trust that the
authors remember to set the output parameter in all cases.

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

@@ -22,7 +22,7 @@ static int node_vtable_get_userdata(
                 sd_bus_error *error) {
 
         sd_bus_slot *s;
-        void *u, *found_u;
+        void *u, *found_u = NULL;
         int r;
 
         assert(bus);