Qemu: cirrus: OOB access issue in mode4and5 write functions

Quick emulator QEMU, compiled with the Cirrus CLGD 54xx VGA Emulator support, is vulnerable to an OOB write access issue. The issue could occur while writing to VGA memory via mode4and5 write functions. A privileged user inside guest could use this flaw to crash the QEMU process resulting in Deni...