[sr-dev] [kamailio/kamailio] nathelper: 'a=rtcp' without IP generates an ERROR (#2768)

[Victor Seva]

### Description
As described at [RFC3605](https://datatracker.ietf.org/doc/html/rfc3605#section-2.1)
```
rtcp-attribute =  "a=rtcp:" port  [nettype space addrtype space  connection-address] CRLF
```
So IP info is optional.

### Troubleshooting

#### Log Messages

```
Jun 7 09:55:57 sp2 lb[46796]: ERROR: DEFAULT_ROUTE nathelper [nathelper.c:1796]: nh_extract_mediaip(): no `IP[4|6]' in `a=rtcp' field
Jun 7 09:55:57 sp2 lb[46796]: ERROR: DEFAULT_ROUTE nathelper [nathelper.c:1617]: replace_sdp_ip(): can't extract 'a=rtcp' IP from the SDP
```

#### SIP Traffic

```
INVITE sip:jsung2 at 192.168.88.194;line=8c86b7427b1107f SIP/2.0
Via: SIP/2.0/UDP 192.168.88.194;branch=z9hG4bK4f82.52fb005dac4fe078dd5593a1bebfa33b.0
From: <sip:4316607564835 at xxxx>;tag=40FED3EF-60BDED10000D2F23-71F92700
To: <sip:jsung2 at xxxx>
CSeq: 939 INVITE
Call-ID: CDqyvKE3ryYIRkecp_7cXA..
Max-Forwards: 69
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, MESSAGE, OPTIONS, SUBSCRIBE
P-Asserted-Identity: <sip:4316607564835 at sip.lab.com>
Diversion: <sip:4316607564835 at xxxx>;privacy=off;counter=1
Supported: timer
Session-Expires: 1800
Min-SE: 90
Content-Type: application/sdp
Content-Length: 316
Contact: <sip:btpsh-60baedf4-645b-3 at 192.168.88.194>

v=0
o=Z 1623059728682 1 IN IP4 192.168.88.194
s=Z
c=IN IP4 192.168.88.194
t=0 0
m=audio 20252 RTP/AVP 8 0 9 98 101
a=rtpmap:8 PCMA/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:9 G722/8000
a=rtpmap:98 telephone-event/48000
a=fmtp:98 0-16
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=sendrecv
a=rtcp:20253
```

### Possible Solutions

fix for nathelper will follow

### Additional Information

```
version: kamailio 5.5.0 (x86_64/linux)
```

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https://github.com/kamailio/kamailio/issues/2768
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