JACK: re-create backend when re-connecting to the server

This updates the client-name when re-connecting to the
server, which is apparently useful with NSM.

libs/backends/jack/jack_api.cc

@@ -58,6 +58,7 @@ instantiate (const std::string& arg1, const std::string& arg2)
 {
 	try {
 		jack_connection.reset (new JackConnection (arg1, arg2));
+		backend.reset ();
 	} catch (...) {
 		return -1;
 	}