xen: check for zero sized area when invalidating memory

With the introduction of the linear mapped p2m list setting memory
areas to "invalid" had to be delayed. When doing the invalidation
make sure no zero sized areas are processed.

Signed-off-by: Juegren Gross <jgross@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>