Fix an obvious bug

And make it easy not to make the same mistake

src/cpp/server/thread_pool.cc

@@ -41,8 +41,10 @@ ThreadPool::ThreadPool(int num_threads) {
       for (;;) {
         std::unique_lock<std::mutex> lock(mu_);
         // Wait until work is available or we are shutting down.
-        if (!shutdown_ || callbacks_.empty())
-          cv_.wait(lock, [=]() { return shutdown_ || !callbacks_.empty(); });
+        auto have_work = [=]() { return shutdown_ || !callbacks_.empty(); };
+        if (!have_work()) {
+          cv_.wait(lock, have_work);
+        }
         // Drain callbacks before considering shutdown to ensure all work
         // gets completed.
         if (!callbacks_.empty()) {