msvcrt: Fix strtok_s for an empty string.

[Piotr Caban]

Hi Bernhard,

On 08/18/16 03:48, Bernhard Übelacker wrote:
> +    old_handler = p_set_invalid_parameter_handler(test_strtok_s_handler);
I'm not sure if it makes sense to set invalid parameter handler taking 
in account it's never called. If you don't set it and it's called the 
tests will crash.

> +    context = (char*)0xdeadbeef;
> +    strret = p__mbstok_s( test, "/", &context);
This will cause compilation warnings (signedness mismatch).

Thanks,
Piotr