x86: replace bad ASSERT() in xenmem_add_to_physmap_one()

x86: replace bad ASSERT() in xenmem_add_to_physmap_one()

There are no locks being held, i.e. it is possible to be triggered by
racy hypercall invocations. Subsequent code doesn't really depend on the
checked values, so this is not a security issue.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>