go/types, types2: don't panic during interface completion

go/types, types2: don't panic during interface completion

It should be possible for the importer to construct an invalid
interface, as would have been produced by type checking.

Fixes #61737

Change-Id: I72e063f4f1a6205d273a623acce2ec08c34c3cc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/515555
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Olif Oftimis <oftimisolif@gmail.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>