CVE-2008-4539: fix a heap overflow in Cirrus emulation

The code in hw/cirrus_vga.c has changed a lot between CVE-2007-1320 has
been announced and the patch has been applied. As a consequence it has
wrongly applied and QEMU is still vulnerable to this bug if using VNC.

(noticed by Jan Niehusmann)

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5587 c046a42c-6fe2-441c-8c8c-71466251a162
