pulse-server: use object serial as index

Use the lower 32 bits of the object serial as the index. When there is
an overflow, use an invalid index, which will probably result in a
protocol error.

src/modules/module-protocol-pulse/manager.h

@@ -75,6 +75,7 @@ struct pw_manager_param {
 
 struct pw_manager_object {
 	struct spa_list link;           /**< link in manager object_list */
+	uint64_t serial;
 	uint32_t id;
 	uint32_t permissions;
 	const char *type;