CVE-2017-13688/OLSR: Do bounds checks before we fetch data.

[tcpdump] / tests / rtp-seg-fault-1.out

IP (tos 0x0, ttl 255, id 158, offset 0, flags [DF], proto UDP (17), length 37, bad cksum d7e0 (->9cf8)!)
    208.21.2.184.1512 > 10.1.1.99.53: udp/rtp 57323 c31 +* 4652 3815804996 [|rtp]