math/big: Rat.SetString may consume large amount of RAM and crash [1.16 backport]

[gopherbot]

@katiehockman requested issue #50699 to be considered for backport to the next 1.16 minor release.

@gopherbot please backport to 1.17 and 1.16 as this is a security issue.

[gopherbot]

Change https://golang.org/cl/381337 mentions this issue: [release-branch.go1.16] math/big: prevent overflow in (*Rat).SetString

[dmitshur]

Approving as a fix to a security issue. This backport applies to both 1.17 (#50701) and 1.16 (this issue).

[gopherbot]

Closed by merging 07ee9e6 to release-branch.go1.16.