crypto/elliptic: IsOnCurve returns true for invalid field elements [1.16 backport]

[gopherbot]

@FiloSottile requested issue #50974 to be considered for backport to the next 1.16 minor release.

@gopherbot please open backport issues for this security fix. /cc @golang/security

[gopherbot]

Change https://golang.org/cl/382457 mentions this issue: [release-branch.go1.16] crypto/elliptic: make IsOnCurve return false for invalid field elements

[gopherbot]

Change https://golang.org/cl/382855 mentions this issue: [release-branch.go1.16] crypto/elliptic: make IsOnCurve return false for invalid field elements

[gopherbot]

Closed by merging 6b3e741 to release-branch.go1.16.