go/types: nil pointer dereference in Alias.Underlying() [1.22 backport]

[gopherbot]

@findleyr requested issue #65455 to be considered for backport to the next 1.22 minor release.

@gopherbot please consider this for backport to 1.22, it's a deterministic panic with a simple fix.

[gopherbot]

Change https://go.dev/cl/564356 mentions this issue: [release-branch.go1.22] go/types, types2: ensure that Alias.actual is set in NewAlias

[gopherbot]

Closed by merging 3b71998 to release-branch.go1.22.