math/big: index out of range in Float.GobDecode [1.17 backport]

[gopherbot]

@rolandshoemaker requested issue #53871 to be considered for backport to the next 1.17 minor release.

@gopherbot please open backports, this is a security issue.

[gopherbot]

Change https://go.dev/cl/419814 mentions this issue: [release-branch.go1.17] math/big: check buffer lengths in GobDecode

[gopherbot]

Closed by merging 703c8ab to release-branch.go1.17.