Fix an unicode issue with amx_SetStringOld.

Issue found by Nextra, while playing with ```contain``` native and noticed it was not working properly with string containing unicode characters. It's because of a wrong data type conversion where signed char negative values will give ```0xffff*``` resulting failed check e.g. ```0xffffc2 == c2```.

This is the same issue with ```set_amxstring```, which has been fixed.

```amx_SetStringOld``` has actually a param to handle this situation. 
Proposed fix is to use enable last param to use ```wchar``` conversion instead.