[release-branch.go1.16] math/big: check for excessive exponents in Rat.SetString

Found by OSS-Fuzz https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=33284

Thanks to Emmanuel Odeke for reporting this issue.

Updates #45910
Fixes #46306
Fixes CVE-2021-33198

Change-Id: I61e7b04dbd80343420b57eede439e361c0f7b79c
Reviewed-on: https://go-review.googlesource.com/c/go/+/316149


Trust: Robert Griesemer <gri@golang.org>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
(cherry picked from commit 6c591f79)
Reviewed-on: https://go-review.googlesource.com/c/go/+/321832


Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>