compiler: fix crash on struct that embeds pointer type

compiler: fix crash on struct that embeds pointer type

    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