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
Note that indirect GLX is now disbled by default.
Tested by naddy@, jsg@ & kettenis@
Tested by at least ajacoutot@, dcoppa@ & jasper@