rangeset: make inquiry functions tolerate NULL inputs

rangeset: make inquiry functions tolerate NULL inputs

Rather than special casing the ->iomem_caps check in x86's
get_page_from_l1e() for the dom_xen case, let's be more tolerant in
general, along the lines of rangeset_is_empty(): A never allocated
rangeset can't possibly contain or overlap any range.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>