Fixed threadpool bug when running on single core systems.

*Thanks to freshman for reporting bug.

src/cryptonote_core/blockchain.cpp

@@ -2009,10 +2009,13 @@ bool Blockchain::check_tx_inputs(const transaction& tx, uint64_t* pmax_used_bloc
     boost::thread_group threadpool;
 
     std::auto_ptr < boost::asio::io_service::work > work(new boost::asio::io_service::work(ioservice));
+	if(threads > 1)
+	{
 	    for (int i = 0; i < threads; i++)
 	    {
 	        threadpool.create_thread(boost::bind(&boost::asio::io_service::run, &ioservice));
 	    }
+	}
 	
 #define KILL_IOSERVICE()  \
         if(threads > 1) \