Fixed crash in AMX_GetGlobal() when using JIT.

It's basically the same fix as 2 years ago:
https://github.com/Y-Less/YSI/commit/c5065d796575fca9d19128bd33ed04b04f0c2d41
Merge pull request #30 from Zeex/YSI.tl

Fixed crash in AMX_GetGlobal() when using JIT.