fixes an assertion failure in qemu network packet processing, which can
lead to DOS'ing the qemu process on the host. this affects 'e1000e' and
'vmxnet3' network devices.
patch is cherry-picked from the commit mentioned in the oss-security email.
more info on oss-security [0]
[0]: https://www.openwall.com/lists/oss-security/2020/08/10/1
Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
