Print warning when I execute my own extension function

Hi hackers,
I've read in this blog (
http://big-elephants.com/2015-10/writing-postgres-extensions-part-i/)
and I wrote an extension about base36_encode with c code
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.