Worker thread of Client class should not sleep when stopping

https://github.com/ethereum/cpp-ethereum/pull/4948/files#r180332663

When there is a request to stop the thread, the thread should not enter
the sleeping state to wait for something else.

libethereum/Client.cpp

@@ -693,7 +693,7 @@ void Client::doWork(bool _doWait)
         isSealed = m_working.isSealed();
     // If the block is sealed, we have to wait for it to tickle through the block queue
     // (which only signals as wanting to be synced if it is ready).
-    if (!m_syncBlockQueue && !m_syncTransactionQueue && (_doWait || isSealed))
+    if (!m_syncBlockQueue && !m_syncTransactionQueue && (_doWait || isSealed) && isWorking())
     {
         std::unique_lock<std::mutex> l(x_signalled);
         m_signalled.wait_for(l, chrono::seconds(1));