security: fix CVE-2021-34558 [1.16 backport]

[gopherbot]

@FiloSottile requested issue #47143 to be considered for backport to the next 1.16 minor release.

@gopherbot please file backport issues for this security fix.

[gopherbot]

Change https://golang.org/cl/334029 mentions this issue: [release-branch.go1.16] crypto/tls: test key type when casting

[gopherbot]

Closed by merging 58bc454 to release-branch.go1.16.