nsysnet: Check for null timeout in select call

[deReeperJosh]

After bisecting an error with Skylanders Superchargers and Imaginators, I found that after this commit bab1616 was added, the nsysnet select method was being called with a non-null write fdset, but a null timeout. I have added a null check to the code to return early if this timeOut is null. Fixes #1321

[Exzap]

Thanks for tracing this down to select(). I am not happy with the implementation because, according to man pages, a NULL timeout is an infinite timeout. This is also doesn't simulate a zero timeout and instead quits right away before checking any of the sockets. Whether Wii U follows any of these behaviors is another question.

But at the end of the day this is acceptable as a workaround and our select() implementation is all sorts of hacky already anyway. So I will merge this as-is and look into fixing up the select implementation when I get a chance. Thanks again for looking into this!