icmp: fix miscalculation on multipart message bodies

Also adds test cases for the Len method of MessageBody interface.

Fixes golang/go#13141.

Change-Id: I8ab9e38727104ca094dfdb3020e8d42e611911e0
Reviewed-on: https://go-review.googlesource.com/16616
Reviewed-by: Ian Lance Taylor <iant@golang.org>
5 files changed