Fix keeper-bench in case of error during scheduling a thread

Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>

utils/keeper-bench/Runner.cpp

@@ -167,6 +167,7 @@ void Runner::runBenchmark()
     }
     catch (...)
     {
+        shutdown = true;
         pool.wait();
         throw;
     }