go/types: fix typo causing loss of embedded interfaces

Simplified the code per prior suggestion to avoid that
kind of error in the first place.

Also: Fix subtle error in Interface.Complete where an
interface may have ended up incomplete if both the list
of methods and the list of embedded interfaces was nil.

Expanded existing test to cover all these cases.

Fixes golang/go#25577

Change-Id: If8723a8b0c4570f02b3dadfa390f96dd98ce11c8
Reviewed-on: https://go-review.googlesource.com/114504


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>