fixed issue with strokes made using a pen

src/domain/UBGraphicsScene.cpp

@@ -629,8 +629,8 @@ bool UBGraphicsScene::inputDeviceRelease()
 
     if (mCurrentStroke && mCurrentStroke->polygons().empty()){
         delete mCurrentStroke;
+        mCurrentStroke = NULL;
     }
-    mCurrentStroke = NULL;
 
     return accepted;
 }
@@ -2302,10 +2302,12 @@ void UBGraphicsScene::setToolCursor(int tool)
 
     if (mCurrentStroke && mCurrentStroke->polygons().empty()){
         delete mCurrentStroke;
+        mCurrentStroke = NULL;
     }
-    mCurrentStroke = NULL;
+
 }
 
-void UBGraphicsScene::initStroke(){
+void UBGraphicsScene::initStroke()
+{
 	mCurrentStroke = new UBGraphicsStroke();
 }