[release-branch.go1.14] math/big: correct off-by-one access in divBasic

The divBasic function computes the quotient of big nats u/v word by word.
It estimates each word qhat by performing a long division (top 2 words of u
divided by top word of v), looks at the next word to correct the estimate,
then perform a full multiplication (qhat*v) to catch any inaccuracy in the
estimate.

In the latter case, "negative" values appear temporarily and carries
must be carefully managed, and the recursive division refactoring
introduced a case where qhat*v has the same length as v, triggering an
out-of-bounds write in the case it happens when computing the top word
of the quotient.

Fixes #37501

Change-Id: I15089da4a4027beda43af497bf6de261eb792f94
Reviewed-on: https://go-review.googlesource.com/c/go/+/221980


Reviewed-by: Robert Griesemer <gri@golang.org>
(cherry picked from commit ac1fd419)
Reviewed-on: https://go-review.googlesource.com/c/go/+/227877
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>