[release-branch.go1.17] math/big: check buffer lengths in GobDecode
In Float.GobDecode and Rat.GobDecode, check buffer sizes before
indexing slices.
Updates #53871
Fixes #54094
Change-Id: I1b652c32c2bc7a0e8aa7620f7be9b2740c568b0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/417774
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tatiana Bradley <tatiana@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
(cherry picked from commit
055113ef364337607e3e72ed7d48df67fde6fc66)
Reviewed-on: https://go-review.googlesource.com/c/go/+/419814
Reviewed-by: Julie Qiu <julieqiu@google.com>