package/slirp: add upstream security fix for CVE-2020-29129 / CVE-2020-29130

While processing ARP/NCSI packets in 'arp_input' or 'ncsi_input'
routines, ensure that pkt_len is large enough to accommodate the
respective protocol headers, lest it should do an OOB access.
Add check to avoid it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>