mgw: accept zero-length RTP payload as BFI Mainline OsmoBTS now has an option (rtp continuous-streaming) that causes it to emit an RTP packet every 20 ms without gaps, sending a BFI marker in the form of zero-length RTP payload when it has nothing else to send. These codec-independent BFI markers don't indicate TAF, but this provision is a good start. Accept this BFI packet format in themwi-mgw.

/*
 * Here we define struct uas_parse_hdrs, a structure that captures
 * the 5 essential header fields that must be present in every
 * received request in order for us to be able to respond to it.
 */

struct uas_parse_hdrs {
	char		*from;
	char		*to;
	char		*call_id;
	unsigned	cseq_num;
	char		*via;
	char		*error_field;
};