dix: integer overflow in RegionSizeof() [CVE-2014-8092 3/4]

RegionSizeof contains several integer overflows if a large length
value is passed in.  Once we fix it to return 0 on overflow, we
also have to fix the callers to handle this error condition

v2: Fixed limit calculation in RegionSizeof as pointed out by jcristau.

Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Julien Cristau <jcristau@debian.org>