Fix destructor race in Job

src/ripple/core/impl/Job.cpp

@@ -80,6 +80,10 @@ void Job::doJob ()
     m_loadEvent->reName (mName);
 
     mJob (*this);
+
+    // Destroy the lambda, otherwise we won't include
+    // its duration in the time measurement
+    mJob = std::function<void(Job&)>();
 }
 
 void Job::rename (std::string const& newName)