cmd/cover: fix sorting of profile segment boundaries

If a span of coverable code is empty (e.g. an empty select clause)
then there will be two Boundary values with the same offset. In that
case, the starting Boundary needs to come first so that the generated
HTML output will open the <span> tag before it tries to close it.

Change-Id: Ib44a8b7c36ae57757c18b6cceb7a88ffa4e95394
Reviewed-on: https://go-review.googlesource.com/114855


Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>