port=0 in SDP offer sent by w4a (m=audio or m=video lines)

```
What steps will reproduce the problem?
1. Make an audio/video call from w4a to a legacy device (through Asterisk)
2.
3.

What is the expected output? What do you see instead?

W4A should always send a SDP with a non-zero ports for m=audio and m=video
lines.
Instead:
- sometimes, w4a sends a SDP offer with port=0 on the m=video line:
m=video 0 RTP/SAVPF 125 104 105 121 103 102 126 34

- sometimes, w4a sends a SDP offer with port=0 on the m=audio line:
m=audio 0 RTP/SAVPF 3 8 0 9 97 98 99 101

- sometimes, w4a sends a correct SDP offer:
m=audio 49898 RTP/SAVPF 3 8 0 9 97 98 99 101
m=video 49894 RTP/SAVPF 125 104 105 121 103 102 126 34

- never, w4a sends a SDP offer where port=0 on both m=audio and m=video lines

What version of the product are you using? On what operating system?
Problem has been reproduced with:
- w4a 1.11.745 with Asterisk r372699
- w4a 1.12.756 with Asterisk r373330

Please provide any additional information below.

Problem has been reproduced in Windows 7 and 8.
IPv6 has been deactivated in windows (in registry), it does not solve totally
the issue.
Problem has been reproduced in a PC equiped with a single network interface
(one entry in IPCONFIG), and a PC with multiple network interface (ethernet,
wireless, virtualbox, ...)
```

Original issue reported on code.google.com by `zobo...@gmail.com` on 22 Nov 2012 at 2:41