[sr-dev] [kamailio/kamailio] Wrong return code used by sl_reply_error() (#2802)

[Norbixin]

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### Description

I wanted to change 478 response to 503 in case of t_relay() failure, but leave default behavior in rest error cases. I used part of code from "Example 1.1." inside https://www.kamailio.org/docs/modules/devel/modules/outbound.html. When I get -478 from t_relay() it works fine, but when this code is different sl_reply_erorr() don't use it as it should be. 

This is my part of kamailio.cfg:
```
route[RELAY] {
    if (!t_relay()) {
        switch($rc) {
            case -478:
                xlog("L_WARN","T_Relay returned 478, sending back 503.\n");
                sl_send_reply("503", "Service Unavailable");
                exit;
            default:
                sl_reply_error();
                exit;
        }
    }
    exit;
}
```

When I log $rc value before sl_reply_error() it is valid, but not used by sl_reply_error() as SIP error value, it sends 500 instead.

### Troubleshooting

#### Reproduction

I've added incorrect hostnames with flag 16 to set where the message should be sent and t_relay() returns -478. To check the default case inside switch I changed case -478 to -479.

#### Log Messages

<!--
Check the syslog file and if there are relevant log messages printed by Kamailio, add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site).
-->

```
1(8) ERROR: <core> [core/resolve.c:1731]: sip_hostport2su(): could not resolve hostname: "va4brhino"
 1(8) ERROR: tm [ut.h:309]: uri2dst2(): failed to resolve "va4brhino"
 1(8) ERROR: tm [t_fwd.c:1759]: t_forward_nonack(): failure to add branches
 1(8) ERROR: sl [sl_funcs.c:414]: sl_reply_error(): stateless error reply used: I'm terribly sorry, server error occurred (1/SL)
```

Logs with different route relay
```
route[RELAY] {
    if (!t_relay()) {
        switch($rc) {
            case -479:
                xlog("L_WARN","T_Relay returned 479, sending back 503.\n");
                sl_send_reply("503", "Service Unavailable");
                exit;
            default:
                xlog("L_WARN","Return code: $rc\n");
                sl_reply_error();
                exit;
        }
    }
    exit;
}
```
```
 1(8) ERROR: <core> [core/resolve.c:1731]: sip_hostport2su(): could not resolve hostname: "va4brhino"
 1(8) ERROR: tm [ut.h:309]: uri2dst2(): failed to resolve "va4brhino"
 1(8) ERROR: tm [t_fwd.c:1759]: t_forward_nonack(): failure to add branches
 1(8) WARNING: <script>: Return code: -478
 1(8) ERROR: sl [sl_funcs.c:414]: sl_reply_error(): stateless error reply used: I'm terribly sorry, server error occurred (1/SL)
```


### Additional Information

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

```
version: kamailio 5.5.0 (x86_64/linux)
flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_
DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLOCKLIST, HAVE_RESOLV_RES
ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB
poll method support: poll, epoll_lt, epoll_et, sigio_rt, select.
id: unknown
compiled with gcc 4.8.4
```

* **Operating System**:

<!--
Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...;
Kernel details (output of `uname -a`)
-->

```
Linux v4bkamailio 5.10.25-linuxkit #1 SMP Tue Mar 23 09:27:39 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
```


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