dinput: Added a check for NULL callback in EnumDevices.

[Lucas Zawacki]

> Sometimes apps depend on the crash.  Since your change comes with a
> test case, it looks reasonable to me, although you might check the
> specific HRESULT rather than just FAILED.

Exactly, it's not a crash on Windows. I'll resend it with an explicit test.

I was thinking of this more as "janitorial" work as opposed to fixing
a buggy app, maybe my comment worded it badly.