[sr-dev] [kamailio/kamailio] crash in slow timer (#1897)

[lazedo]

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### Description

slow_timer proc exited with crash, kamailio kept running but tm transaction kept accumulating which will lead to shared memory exhaustion and crash.

in line 1131 `ret=tl->f(*ticks, tl, tl->data);` f is null, reason of crash
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### Troubleshooting

#### Reproduction

happens from time to time, not a way to reliably reproduce it.
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#### Debugging Data

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```
#0  0x0000000000000000 in ?? ()
#1  0x000000000049f648 in slow_timer_main () at core/timer.c:1131
#2  0x0000000000424b84 in main_loop () at main.c:1685
#3  0x000000000042b9d3 in main (argc=16, argv=0x7ffe568bcd08) at main.c:2663
(gdb) frame 1
#1  0x000000000049f648 in slow_timer_main () at core/timer.c:1131
1131						ret=tl->f(*ticks, tl, tl->data);
(gdb) list
1126	#ifdef TIMER_DEBUG
1127					tl->expires_no++;
1128	#endif
1129					SET_RUNNING_SLOW(tl);
1130					UNLOCK_SLOW_TIMER_LIST();
1131						ret=tl->f(*ticks, tl, tl->data);
1132						/* reset the configuration group handles */
1133						cfg_reset_all();
1134						if (ret==0){
1135							/* one shot */
(gdb) p *tl
$1 = {next = 0x0, prev = 0x0, expire = 938928580, initial_timeout = 80, data = 0x0, f = 0x0, flags = 768, slow_idx = 45488}
(gdb) info locals
n = 12
ret = 4294967295
tl = 0x7fb2d3c860d0
i = 432
__func__ = "slow_timer_main"

```

#### Log Messages

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#### SIP Traffic

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### Possible Solutions

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### Additional Information

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

```
5.1.7
```

* **Operating System**:

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```
centos 7
```


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