table: simplify StrategyChoice with C++11 features

refs #2100

Change-Id: If0082c8561d706df86e92c14cb87762962ff200b
diff --git a/daemon/mgmt/strategy-choice-manager.cpp b/daemon/mgmt/strategy-choice-manager.cpp
index 962349d..e93353b 100644
--- a/daemon/mgmt/strategy-choice-manager.cpp
+++ b/daemon/mgmt/strategy-choice-manager.cpp
@@ -162,7 +162,9 @@
   if (m_strategyChoice.insert(prefix, selectedStrategy))
     {
       NFD_LOG_DEBUG("strategy-choice result: SUCCESS");
-      parameters.setStrategy(*m_strategyChoice.get(prefix));
+      auto currentStrategyChoice = m_strategyChoice.get(prefix);
+      BOOST_ASSERT(currentStrategyChoice.first);
+      parameters.setStrategy(currentStrategyChoice.second);
       setResponse(response, 200, "Success", parameters.wireEncode());
     }
   else