Xi: integer overflow and unvalidated length in (S)ProcXIBarrierReleasePointer

[jcristau: originally this patch fixed the same issue as commit
 211e05ac "Xi: Test exact size of XIBarrierReleasePointer", with the
 addition of these checks]

This addresses CVE-2017-12179

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Signed-off-by: Nathan Kidd <nkidd@opentext.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>
(cherry picked from commit d088e3c1)