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

### Troubleshooting

#### Reproduction

happens from time to time, not a way to reliably reproduce it. <!-- If the issue can be reproduced, describe how it can be done. -->

#### Debugging Data

<!-- If you got a core dump, use gdb to extract troubleshooting data - full backtrace, local variables and the list of the code at the issue location.

gdb /path/to/kamailio /path/to/corefile bt full info locals list

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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**:

<!-- 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`) -->

``` centos 7 ```