[sr-dev] [kamailio/kamailio] cscf_get_ue_via does not return UE via (#2864)

[ErhanOnur]

### Description

I put a dispatcher between PCSCF and UE.  When I tried to register,  Pcscf's save function saved dispatcher's via IP and via port  instead of UE's via IP and via port.

When I checked save function of ims_registrar_pcscf module , I saw that cscf_get_ue_via function is used for that. And It gives wrong via value. Instead of giving last via , it gives the first via in the via list.

Here is the function:

https://github.com/kamailio/kamailio/blob/master/src/lib/ims/ims_getters.c

```
/**
 * Looks for the UE Via in First Via header if its a request
 * or in the last if its a response and returns its body
 * @param msg - the SIP message
 * @returns the via of the UE
 */
struct via_body* cscf_get_ue_via(struct sip_msg *msg)
{
	struct via_body *vb=0;

	if (msg->first_line.type==SIP_REQUEST) vb = cscf_get_first_via(msg,0);
	else vb = cscf_get_last_via(msg);

	if (!vb) return 0;

	if (vb->port == 0) vb->port=5060;
	return vb;	
}
```


### Troubleshooting

I corrected the code as below:

```
/**
 * Looks for the UE Via in Last Via header and returns its body
 * @param msg - the SIP message
 * @returns the via of the UE
 */
struct via_body* cscf_get_ue_via(struct sip_msg *msg)
{
	struct via_body *vb=0;

	vb = cscf_get_last_via(msg);

	if (!vb) return 0;

	if (vb->port == 0) vb->port=5060;
	return vb;	
}
```

#### Reproduction

Try to put a dispatcher between PCSCF and UE. save function of ims_registrar_pcscf module saves dispatcher's IP and port as UE's IP and port.

#### Debugging Data


```
(paste your debugging data here)
```

#### Log Messages



```
(paste your log messages here)
```

#### SIP Traffic

<!--
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-->

```
(paste your sip traffic here)
```

### Possible Solutions

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If you found a solution or workaround for the issue, describe it. Ideally, provide a pull request with a fix.
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### Additional Information

  * **Kamailio Version** - output of `kamailio -v`

```
(paste your output here)
```

* **Operating System**:

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-->

```
(paste your output here)
```


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