res_pjsip_session.c: Correctly format SDP connection addresses.

res_pjsip_session.c: Correctly format SDP connection addresses.

Resolves a regression identified by @justinludwig involving the
rendering of IPv6 addresses in outgoing SDP.

Also updates `media_address` on PJSIP endpoints so that if we are able
to parse the configured value as an IP we store it in a format that we
can directly use later. Based on my reading of the code it appeared
that one could configure `media_address` as:

```
[foo]
type = endpoint
...
media_address = [2001:db8::]
```

And that value would be blindly copied into the outgoing SDP without
regard to its format.

Fixes #541