This fixes two critical security vulnerabilities that lead to an
out-of-bounds memory access with a crafted client framebuffer update
request packet. The dimensions of the update from the packet are checked
to ensure that they are within the screen dimensions.

Thanks to Kevin Chen from the Bitblaze group for the reports in bugs
641802 and 641803. The CVE identifiers for these vulnerabilities are
CVE-2011-0904 and CVE-2011-0905.