usbstor_new: crash when unloading driver on x64

This is what happens:

• usbstor is enumerated
• IopStartDevice2 is called
• usbstor fails to initialize (unrelated x64 issue)
• IopStartDevice2 calls IopRemoveDevice, passing an uninitialized DEVICE_NODE structure
• IopRemoveDevice calls IopQueueTargetDeviceEvent
• IopQueueTargetDeviceEvent calls RtlAppendUnicodeStringToString, passing the uninitialized DeviceIds unicode string.