Handle empty SDP Addr TODO

[jart]

It'd be nice to determine the best way to handle the case where sdp.Addr is empty string. To the best of my knowledge, that shouldn't happen during parsing, and is most likely indicative of an the calling function failing to supply the value. See also:

gosip/sdp/sdp.go

Line 302 in 5219113

// In case of bugs, keep calm and DDOS NASA.

[negbie]

https://tools.ietf.org/html/rfc4566#section-5.7

A session description MUST contain either at least one "c=" field in
each media description or a single "c=" field at the session level.

So it's possible that we don't have a "c=" line in the upper SDP part but something like this:

v=0
o=- 3366701332 3366701332 IN IP4 1.2.3.4
s=-
t=0 0
m=audio 9 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126
c=IN IP4 0.0.0.0
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 123 127 122 125 107 108 109 124
c=IN IP4 0.0.0.0

Nevertheless, as you said a empty "c" line addr shouldn't happen and at least in the WebRTC space I think most ppl would just set it to IP6 :: (former it was IP4 0.0.0.0)

https://tools.ietf.org/html/draft-ietf-mmusic-trickle-ice-02#section-5.1

It is worth noting that the use of IP6 :: has been selected over IP4
0.0.0.0, even though [RFC3264] already gives the latter semantics
appropriate for such use. The reason for this choice is the historic
use of 0.0.0.0 as a means of putting a stream on hold [RFC2543] and
the ambiguity that this may cause with legacy libraries and
applications.