Fix runtime error in variadic functions that return strings

This fixes a bug where returning a string from a variadic function caused
an invalid memory access error during runtime. It seems like they forgot
to update existing string return code for variadic functions.

See 11) here: http://forum.sa-mp.com/showthread.php?t=355877

--------- test code --------

native print(const s[]);

stock f(...)
{
	new a, b, c;
    new str[] = "hello";
    return str;
}

main() {
	print(f(1, 2, 3));
}

----- end of test code -----