Fix stack underflow with multiple plugins

This fixes a bug in the amx_Exec() hook where the parameters were
popped off the stack multiple times if running multiple GDK based
plugins and at least one of them is using the static/amalgamation
build.

Now only the first plugin (the one that calls the real amx_Exec) does
the stack cleanup. The others know about this by looking at both
the index and error code.