for bug #237, never sleep when thread interval is 0.

@@ -129,9 +129,9 @@ public:
 };
 
 /**
- * the publish recv thread got message and callback the source method to process message.
+* the publish recv thread got message and callback the source method to process message.
 * @see: https://github.com/winlinvip/simple-rtmp-server/issues/237
- */
+*/
 class SrsPublishRecvThread : public ISrsMessageHandler
 {
 private:

@@ -179,7 +179,11 @@ failed:
             break;
         }
         
-        st_usleep(cycle_interval_us);
+        // to improve performance, donot sleep when interval is zero.
+        // @see: https://github.com/winlinvip/simple-rtmp-server/issues/237
+        if (cycle_interval_us > 0) {
+            st_usleep(cycle_interval_us);
+        }
     }
     
     handler->on_thread_stop();