Fix crash on shutdown

Sometimes we insert NULL into the client object map to mark the object
freed. Now that the map also returns those NULL's, don't try to free
them.
Small cleanups
Make some port methods private.

src/pipewire/private.h

@@ -372,6 +372,17 @@ struct pw_node_factory {
 	void *user_data;
 };
 
+/** Create a new port \memberof pw_port
+ * \return a newly allocated port */
+struct pw_port *
+pw_port_new(enum pw_direction direction,
+	    uint32_t port_id,
+	    struct pw_properties *properties,
+	    size_t user_data_size);
+
+/** Get the user data of a port, the size of the memory was given \ref in pw_port_new */
+void * pw_port_get_user_data(struct pw_port *port);
+
 /** Add a port to a node \memberof pw_port */
 bool pw_port_add(struct pw_port *port, struct pw_node *node);