The type verification code that enforces rules about the types of
    embedded struct fields was not properly handling the case where the
    pointed-to type is a pointer type, e.g.
    
      type s *struct{ C int }
      type t struct{ *s }
    
    which is illegal according to the spec. Tweak the verifier to catch
    this case, and add a guard in the lowering pass to make sure that we
    don't crash on invalid accesses to field "C" in type "t" above.
    
    Fixes golang/go#22050
    
    Reviewed-on: https://go-review.googlesource.com/66530

From-SVN: r253236