hw/xfree86: unvalidated lengths

This addresses:
CVE-2017-12180 in XFree86-VidModeExtension
CVE-2017-12181 in XFree86-DGA
CVE-2017-12182 in XFree86-DRI

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 1b1d4c04)