security: fix CVE-2024-24783 [1.22 backport]

[neild]

Security fix backport for #65390.

[gopherbot]

Change https://go.dev/cl/569235 mentions this issue: [release-branch.go1.22] crypto/x509: make sure pub key is non-nil before interface conversion

[gopherbot]

Closed by merging 337b8e9 to release-branch.go1.22.