slirp: Don't crash on packets from 0.0.0.0/8.

LWIP can generate packets with a source of 0.0.0.0, which triggers an
assertion failure in arp_table_add().  Instead of crashing, simply return
to avoid adding an invalid ARP table entry.

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
