[recovery-netstack] Simplify ICMP logic in one unrealistic edge case

- We previously had an assertion that could be triggered by a remote
  host if they somehow got us to process an IP packet with a 4GB IP
  header
- Since we don't care to support such an extreme edge case, and such
  a case can almost certainly never happen in practice anyway, we
  simply allow our logic to be wrong in that case, and add a comment
  explaining this

Change-Id: I0fce32fee9b48566a9937ea9060efdde5375e26e