xfixes: unvalidated lengths (CVE-2017-12183)

v2: Use before swap (Jeremy Huddleston Sequoia)

v3: Fix wrong XFixesCopyRegion checks (Alan Coopersmith)

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 55caa8b0)