libavformat/dashdec: Fix for ticket 6658 (Dash demuxer segfault)

1 Add function 'resolve_content_path' to propagate the baseURL from
upper level nodes.
 * if no baseURL is available, the path of mpd file will be set as the baseURL.
2 Remove checking for newly established connection.
3 Establish the communication protocol in each connection rather than
 applying one protocol to all connection.
