Initialize background model in background subtraction KNN.

The method does cvCheckPixelBackgroundNP (which reads bgmodel)
before it is ever updated (in cvUpdatePixelBackgroundNP). This
initialization is thus needed to avoid reads of unitialized values.

modules/video/src/bgfg_KNN.cpp

@@ -122,7 +122,7 @@ public:
     //! computes a background image which are the mean of all background gaussians
     virtual void getBackgroundImage(OutputArray backgroundImage) const;
 
-    //! re-initiaization method
+    //! re-initialization method
     void initialize(Size _frameSize, int _frameType)
     {
     frameSize = _frameSize;
@@ -137,6 +137,7 @@ public:
     // for each sample of 3 speed pixel models each pixel bg model we store ...
     // values + flag (nchannels+1 values)
     bgmodel.create( 1,(nN * 3) * (nchannels+1)* size,CV_8U);
+    bgmodel = Scalar::all(0);
 
     //index through the three circular lists
     aModelIndexShort.create(1,size,CV_8U);