goprotobuf: Don't treat groups as messages.

They no longer get XXX_unrecognized fields, nor satisfy the proto.Message interface.

R=nigeltao, r
CC=golang-dev
http://codereview.appspot.com/6829043
diff --git a/proto/equal.go b/proto/equal.go
index 8c26e75..11ea6ed 100644
--- a/proto/equal.go
+++ b/proto/equal.go
@@ -126,7 +126,13 @@
 		}
 	}
 
-	u1 := v1.FieldByName("XXX_unrecognized").Bytes()
+	// Groups don't have XXX_unrecognized.
+	uf := v1.FieldByName("XXX_unrecognized")
+	if !uf.IsValid() {
+		return true
+	}
+
+	u1 := uf.Bytes()
 	u2 := v2.FieldByName("XXX_unrecognized").Bytes()
 	if !bytes.Equal(u1, u2) {
 		return false