lp: keep LpPacket parsed at all times

refs #4156

Change-Id: I21701ab0da2abacc8a84d6ee1a72c2b414e6669a
diff --git a/tests/unit-tests/lp/packet.t.cpp b/tests/unit-tests/lp/packet.t.cpp
index 7929487..c173ed2 100644
--- a/tests/unit-tests/lp/packet.t.cpp
+++ b/tests/unit-tests/lp/packet.t.cpp
@@ -125,9 +125,13 @@
   BOOST_CHECK_NO_THROW(packet.add<FragmentField>(std::make_pair(frag.begin(), frag.end())));
   BOOST_CHECK_NO_THROW(packet.add<FragIndexField>(0));
   BOOST_CHECK_NO_THROW(packet.add<AckField>(2));
+  BOOST_REQUIRE_NO_THROW(packet.wireEncode());
   BOOST_CHECK_NO_THROW(packet.add<FragCountField>(1));
+  BOOST_REQUIRE_NO_THROW(packet.wireEncode());
   BOOST_CHECK_NO_THROW(packet.add<AckField>(4));
+  BOOST_REQUIRE_NO_THROW(packet.wireEncode());
   BOOST_CHECK_NO_THROW(packet.add<AckField>(3));
+  BOOST_REQUIRE_NO_THROW(packet.wireEncode());
   Block wire;
   BOOST_REQUIRE_NO_THROW(wire = packet.wireEncode());
   BOOST_CHECK_EQUAL_COLLECTIONS(expectedBlock, expectedBlock + sizeof(expectedBlock),