dix: integer overflow in ProcPutImage() [CVE-2014-8092 1/4]

ProcPutImage() calculates a length field from a width, left pad and depth
specified by the client (if the specified format is XYPixmap).

The calculations for the total amount of memory the server needs for the
pixmap can overflow a 32-bit number, causing out-of-bounds memory writes
on 32-bit systems (since the length is stored in a long int variable).

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>