xen: fix potential integer overflow in queue_reply

When len is greater than UINT_MAX - sizeof(*rb), in next allocation,
it can overflow integer range and allocates small size of heap.
After that, memcpy will overflow the allocated heap.
Therefore, it needs to check the size of given length.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>