Fixed msg_data assertion in pjsua_acc_send_request() API

[sauwming]

According to the doc of pjsua_acc_send_request():

 * @param msg_data      Optional headers etc. to be added to the outgoing
 *                      request, or NULL if no custom header is desired.

But we raise assertion if no msg_data is provided. Furthermore, we use msg_data->target_uri as destination while dest_uri parameter itself is unused.