[sr-dev] [kamailio/kamailio] free_fixup functions in cmd_export_t are not called (Issue #3814)

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### Description It seems that the fixup_free_* functions are not getting called when used with:

``` static cmd_export_t cmds[] = { {"acc_log_request", (cmd_function)w_acc_log_request, 1, acc_fixup, free_acc_fixup, ANY_ROUTE}, // ... {0, 0, 0, 0, 0, 0} };

static int free_acc_fixup(void **param, int param_no) { LM_CRIT("acc free fixup = %d\n", param_no);

if(*param) { pkg_free(*param); *param = 0; } return 0; } ```

Nothing is getting logged from the function. Is this expected not to print in free_fixup due to the destruction of kamailio and the logging is not reliable?

I also used gdb and added some breakpoints in `fixup` and `free_fixup` functions in some other modules (file_out) and the breakpoint was only found in `fixup` but not in `free_fixup`.

When is the `free_fixup` supposed to be called, at kamailio destruction or after fixing it and before the main loop starts?

<!-- Explain what you did, what you expected to happen, and what actually happened. -->

### Troubleshooting

#### Reproduction

<!-- If the issue can be reproduced, describe how it can be done. --> Use any module that has a custom (maybe also provided from core) `free_fixup` function, and monitor if `free_fixup` is called using debug mode.

#### 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). -->

``` Apr 15 15:52:03 app01 kamailio[812169]: INFO: <core> [core/mem/q_malloc.c:402]: qm_malloc(): qm_malloc(0x7fcdcab18010, 56) called from file_out: file_out.c: fo_fixup_str_index(287) Apr 15 15:52:03 app01 kamailio[812169]: INFO: <core> [core/mem/q_malloc.c:449]: qm_malloc(): qm_malloc(0x7fcdcab18010, 64) returns address 0x7fcdcacc4280 frag. 0x7fcdcacc4240 (size=64) on 1 -th hit```

`0x7fcdcacc4240` fragment is not found anywhere in the logs reported that it was freed unlike other malloced resources. ```

### Additional Information

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

``` version: kamailio 5.9.0-dev0 (x86_64/linux) 8bc64a flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MMAP, PKG_MALLOC, MEM_JOIN_FREE, 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, TLS_PTHREAD_MUTEX_SHARED ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_SEND_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: 8bc64a compiled on 15:32:09 Apr 15 2024 with gcc 10.5.0 ```

* **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 `lsb_release -a` and `uname -a`) -->

``` No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 20.04.6 LTS Release: 20.04 Codename: focal

5.4.0-176-generic #196-Ubuntu SMP Fri Mar 22 16:46:39 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

```