math/big: fix uint64 overflow in nat.mulRange

math/big: fix uint64 overflow in nat.mulRange

Compute median as a + (b-a)/2 instead of (a + b)/2.
Add additional test cases.

Fixes #65025.

Change-Id: Ib716a1036c17f8f33f51e33cedab13512eb7e0be
Reviewed-on: https://go-review.googlesource.com/c/go/+/554617
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>