tests: stop using deprecated Buffer::buf()

Change-Id: I48b7672098cfe57ccb470a247eed0c80e24cfffe
diff --git a/tests/daemon/face/lp-reliability.t.cpp b/tests/daemon/face/lp-reliability.t.cpp
index 5909737..16fb88f 100644
--- a/tests/daemon/face/lp-reliability.t.cpp
+++ b/tests/daemon/face/lp-reliability.t.cpp
@@ -134,7 +134,7 @@
 
     lp::Packet pkt;
     ndn::Buffer buf(payloadSize);
-    std::memcpy(buf.buf(), &pktNo, sizeof(pktNo));
+    std::memcpy(buf.data(), &pktNo, sizeof(pktNo));
     pkt.set<lp::FragmentField>(make_pair(buf.cbegin(), buf.cend()));
     return pkt;
   }