Fix crash when polling for non-existent Kinects.

If no Kinects are present, crash can result from
stepping beyond end of device list. Now we check
to ensure this does not happen.

modules/highgui/src/cap_openni.cpp

@@ -575,7 +575,12 @@ CvCapture_OpenNI::CvCapture_OpenNI( int index )
 
     // Chose device according to index
     xn::NodeInfoList::Iterator it = devicesList.Begin();
-    for( int i = 0; i < index; ++i ) it++;
+		for( int i = 0; i < index && it!=devicesList.End(); ++i ) it++;
+		if ( it == devicesList.End() )
+		{
+			std::cerr << "CvCapture_OpenNI::CvCapture_OpenNI : Faile
+				return;
+		}
 
     xn::NodeInfo deviceNode = *it;
     status = context.CreateProductionTree( deviceNode, productionNode );