efi: Make for_each_efi_memory_desc_in_map() cope with running on Xen

While commit 55f1ea15

 ("efi: Fix for_each_efi_memory_desc_in_map()
for empty memmaps") made an attempt to deal with empty memory maps, it
didn't address the case where the map field never gets set, as is
apparently the case when running under Xen.

Reported-by:  <lists@ssl-mail.com>
Tested-by:  <lists@ssl-mail.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: <stable@vger.kernel.org> # v4.7+
Signed-off-by: Jan Beulich <jbeulich@suse.com>
[ Guard the loop with a NULL check instead of pointer underflow ]
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>