icmp: improve INVARIANTS check

Actually check the conditions that are enforced by the error checking
code instead of a condition which is

• checking a number to be non-negative instead of positive
• depending on a random number

Perform the checks consistently for ICMPv4 and ICMPv6.

Reviewed by: glebius, rrs, cc
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48001