Print warning when I execute my own extension function

but when I executed a query like this below I got a warning below.

```
postgres=# SELECT base36_encode(123);
WARNING:  problem in alloc set ExprContext: detected write past chunk end in block 0x55fb75334d40, chunk 0x55fb75334d68
WARNING:  problem in alloc set ExprContext: detected write past chunk end in block 0x55fb75334d40, chunk 0x55fb75334d68
 base36_encode
---------------
 3f
(1 row)
```

I don't know what this warning means and how I can fix it. 

Thanks

Dong Wook Lee.