Crashing bug when shutdown gracefully

```
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7ffff7fa7700 (LWP 10742)]
0x000000000051fea9 in MySQL_Thread::~MySQL_Thread (this=0x7ffff48170c0, __in_chrg=<optimized out>) at MySQL_Thread.cpp:603
603                     if (mypolls.myds[i]->myds_type==MYDS_LISTENER) {
```

**How to reproduce?**
Run **proxysql shutdown** in admin interface
