math/big: check for excessive exponents in Rat.SetString

math/big: check for excessive exponents in Rat.SetString

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

Fixes #45910.

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>