icmp: correct the assertion that checks limit + jitter
Fixes: 4399e055ea610cdefa1470ad1ee614dd81ba5e56
Approved by: re (cperciva)
(cherry picked from commit f7c4d12bcd5bd7f7fbf6bf9fa601c47e7f97bc5f)
(cherry picked from commit 1b4561cf4b46ffbbd8851aa40bd0402a6bba8906)