Fix segfault when passing invalid arguments

[JoshuaVandaele]

This is an issue I noticed while working on #13288.

[BhaaLseN]

That seems weird. The Init call is unconditional on top level, so the Shutdown should symmetrically be unconditional too.
What part of Shutdown is it segfaulting on?

[JoshuaVandaele]

UICommon::Shutdown segfaults when GCAdapter::Shutdown calls s_libusb_context->IsValid(), I think because InitControllers is not called until we reach MainWindow

[BhaaLseN]

How about testing s_libusb_context before use in GCAdapter::Shutdown instead, then? I'm not too familiar with the adapter code, but it feels weird to move a seemingly unrelated call around. And it might even hide a different bug that we just happened to avoid so far.