Unvalidated lengths

v2: Add overflow check and remove unnecessary check (Julien Cristau)

This addresses:
CVE-2017-12184 in XINERAMA
CVE-2017-12185 in MIT-SCREEN-SAVER
CVE-2017-12186 in X-Resource
CVE-2017-12187 in RENDER

Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Nathan Kidd <nkidd@opentext.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>
(cherry picked from commit cad5a105)