Don't send manual onStart() if frameproducer's not ready.

lib/units/device/plugins/screen/stream.js

@@ -456,7 +456,16 @@ module.exports = syrup.serial()
         })
 
         broadcastSet.on('insert', function(id) {
-          broadcastSet.get(id).onStart(frameProducer)
+          // If two clients join a session in the middle, one of them
+          // may not release the initial size because the projection
+          // doesn't necessarily change, and the producer doesn't Getting
+          // restarted. Therefore we have to call onStart() manually
+          // if the producer is already up and running.
+          switch (frameProducer.runningState) {
+          case FrameProducer.STATE_STARTED:
+            broadcastSet.get(id).onStart(frameProducer)
+            break
+          }
         })
 
         display.on('rotationChange', function(newRotation) {