Use ndn::Buffer::data() instead of buf()

The latter is considered deprecated.

Change-Id: If92406ccedca391e3551b456491ebb47737f1bc1
diff --git a/src/leaf-container.hpp b/src/leaf-container.hpp
index f08f8f9..95ddc75 100644
--- a/src/leaf-container.hpp
+++ b/src/leaf-container.hpp
@@ -49,7 +49,7 @@
 
     BOOST_ASSERT(buffer->size() > sizeof(std::size_t));
 
-    return *reinterpret_cast<const std::size_t*>(buffer->buf());
+    return *reinterpret_cast<const std::size_t*>(buffer->data());
   }
 };