integer overflow in DMXGetWindowAttributes() [CVE-2013-1992 2/3]

If the server provided screenCount causes integer overflow when
multiplied by the size of each array element, a smaller buffer
would be allocated than the amount of data written to it.

Reported-by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>