Fix Windows XP crash when accessing TLS variables

TLS storage happens to be not initialized for DLLs that access TLS variables
in pre-existing threads (like the main thread of the SA-MP server).

More details in this blog post:

https://blogs.msdn.com/b/oldnewthing/archive/2010/11/22/10094489.aspx?Redirected=true

This fixes issue #17.