math/big: avoid negative slice size in nat.rem

math/big: avoid negative slice size in nat.rem

In a division, normally the answer to N digits / D digits has N-D digits,
but not when N-D is negative. Fix the calculation of the number of
digits for the temporary in nat.rem not to be negative.

Fixes #72043.

Change-Id: Ib9faa430aeb6c5f4c4a730f1ec631d2bf3f7472c
Reviewed-on: https://go-review.googlesource.com/c/go/+/655156
Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>