[release-branch.go1.17] math/big: prevent overflow in (*Rat).SetString

Credit to rsc@ for the original patch.

Thanks to the OSS-Fuzz project for discovering this
issue and to Emmanuel Odeke (@odeke_et) for reporting it.

Updates #50699
Fixes #50701
Fixes CVE-2022-23772

Change-Id: I590395a3d55689625390cf1e58f5f40623b26ee5
Reviewed-on: https://go-review.googlesource.com/c/go/+/379537


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
(cherry picked from commit ad345c26)
Reviewed-on: https://go-review.googlesource.com/c/go/+/381336


Reviewed-by: Filippo Valsorda <filippo@golang.org>