Do manual stack cleanup after fake public calls

This fixes a bug in the previous commit.

It turns out amx_Exec() does a check on index *before* removing parameters
off the AMX stack:

https://github.com/Zeex/samp-plugin-crashdetect/blob/v4.15/src/amx/amx.c#L1811
https://github.com/Zeex/samp-plugin-crashdetect/blob/v4.15/src/amx/amx.c#L1843

So add a check for index == AMX_EXEC_GDK and pop the parameters manually.

Fixes #109.