Round uses r+r < d to decide whether the remainder is
above or below half of d (to decide whether to round up or down).
This is wrong when r+r wraps negative, because it looks < d
but is really > d.

No one will ever care about rounding to a multiple of
d > 2⁶² (about 146 years), but might as well get it right.

Fixes #19807.

Change-Id: I1b55a742dc36e02a7465bc778bf5dd74fe71f7c0
Reviewed-on: https://go-review.googlesource.com/39151
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>