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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D48001
Approved by: re (cperciva)

(cherry picked from commit c9febea3dc8ac8e308e8fc5a2e74dd0313fdc5d2)
(cherry picked from commit bd6a89a6c32a0f5b08182ea06e625a6695f3fbb0)