um: Fix initialization of vector queues

UML vector drivers could derefence uninitialized memory
when cleaning up after a queue allocation failure.

Fixes: 49da7e64

 ("High Performance UML Vector Network Driver")
Cc: <stable@vger.kernel.org>
Reported-by: Dan Capenter <dan.carpenter@oracle.com>
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>