hw/net/net_tx_pkt: Check the payload length

hw/net/net_tx_pkt: Check the payload length

Check the payload length if checksumming to ensure the payload contains
the space for the resulting value.

This bug was found by Alexander Bulekov with the fuzzer:
https://patchew.org/QEMU/20230129053316.1071513-1-alxndr@bu.edu/

The fixed test case is:
fuzz/crash_6aeaa33e7211ecd603726c53e834df4c6d1e08bc

Fixes: e263cd49c7 ("Packet abstraction for VMWARE network devices")
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>