security: fix CVE-2021-34558 [1.15 backport]

[gopherbot]

@FiloSottile requested issue #47143 to be considered for backport to the next 1.15 minor release.

@gopherbot please file backport issues for this security fix.

[gopherbot]

Change https://golang.org/cl/334030 mentions this issue: [release-branch.go1.15] crypto/tls: test key type when casting

[gopherbot]

Closed by merging c77980b to release-branch.go1.15.