util: backport C++17 [[nodiscard]] attribute

Change-Id: I6bdeeaed09a4a5d36647ec967aead47e4461e583
diff --git a/tests/unit/encoding/tlv.t.cpp b/tests/unit/encoding/tlv.t.cpp
index 1810f01..478fd83 100644
--- a/tests/unit/encoding/tlv.t.cpp
+++ b/tests/unit/encoding/tlv.t.cpp
@@ -106,9 +106,10 @@
 {
   boost::input_iterator_archetype<uint8_t> begin, end;
   uint64_t number = readVarNumber(begin, end);
-  uint32_t type = readType(begin, end);;
-  readVarNumber(begin, end, number);
-  readType(begin, end, type);
+  uint32_t type = readType(begin, end);
+  bool ok = readVarNumber(begin, end, number);
+  ok = readType(begin, end, type);
+  static_cast<void>(ok);
 }
 
 static const uint8_t BUFFER[] = {