Avoid a divide-by-zero when bad checksum counters overflow.

A mixture of IP or UDP packets with valid and invalid checksum could
cause {ip,udp}_packets_bad_checksum to wrap around to 0, resulting
in a division by zero.

This is packet.c rev. 1.27 from OpenBSD.

admbugs: 552
Obtained from: OpenBSD
MFC after: 3 days