proto: add more invalid group encoding test cases

I noticed we lacked test coverage for these cases before.

Change-Id: Ic407dc77f6bef8b9d496ea5dacdecbf25abc926a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/703295
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: stapelberg

proto/testmessages_test.go

@@ -1945,6 +1945,33 @@ var testInvalidMessages = []testProto{
 			protopack.Tag{46, protopack.StartGroupType},
 		}.Marshal(),
 	},
+	{
+		desc: "incorrectly terminated repeated group field",
+		decodeTo: []proto.Message{
+			(*testpb.TestAllTypes)(nil),
+			(*testeditionspb.TestAllTypes)(nil),
+			(*testpb.TestAllExtensions)(nil),
+			(*testeditionspb.TestAllExtensions)(nil),
+		},
+		wire: protopack.Message{
+			protopack.Tag{46, protopack.StartGroupType},
+			protopack.Tag{45, protopack.EndGroupType}, // should be 46
+		}.Marshal(),
+	},
+	{
+		desc: "incorrectly double-terminated repeated group field",
+		decodeTo: []proto.Message{
+			(*testpb.TestAllTypes)(nil),
+			(*testeditionspb.TestAllTypes)(nil),
+			(*testpb.TestAllExtensions)(nil),
+			(*testeditionspb.TestAllExtensions)(nil),
+		},
+		wire: protopack.Message{
+			protopack.Tag{46, protopack.StartGroupType},
+			protopack.Tag{46, protopack.EndGroupType},
+			protopack.Tag{46, protopack.EndGroupType}, // extra
+		}.Marshal(),
+	},
 	{
 		desc: "invalid tag varint in map item",
 		decodeTo: []proto.Message{