patch for possible DOS in qemu network packet processing

patch for possible DOS in qemu network packet processing

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>