libxc: Check full range of pfns for xc_dom_pfn_to_ptr

libxc: Check full range of pfns for xc_dom_pfn_to_ptr

Previously, passing a valid pfn but an overly large count to
xc_dom_pfn_to_ptr, and functions which call it, would run off the end
of the pfn array giving undefined behaviour.

It is tempting to change this check to an assert, as no callers should
be providing invalid parameters here.  But this is probably best not
done while frozen for 4.0.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>