cvCalcOpticalFlow throws an error is status parameter is NULL (Bugfix #2526)

If I call cvCalcOpticalFlow with the status parameter set to zero, I get the following error:

OpenCV Error: Null pointer (create() called for the missing output array) in create, file /Users/jack/src/OpenCV-2.4.1/modules/core/src/matrix.cpp, line 1461

I was able to fix this by changing this line: (in modules/video/src/lkpyramid.cpp)

    cv::calcOpticalFlowPyrLK( A, B, ptA, ptB, status ? cv::_OutputArray(st) : cv::_OutputArray(),
                              error ? cv::_OutputArray(err) : cv::_OutputArray(),
                              winSize, level, criteria, flags);

to this:

    cv::calcOpticalFlowPyrLK( A, B, ptA, ptB, cv::_OutputArray(st),
                              error ? cv::_OutputArray(err) : cv::_OutputArray(),
                              winSize, level, criteria, flags);

so that it uses the cv::Mat st which exists only in this scope if none is provided.

I don't have any experience using _OutputArray directly, so I'm not sure if this is the best way to do it.

It might be worth doing the same thing for the error parameter, I figured I'd change as little as possible.

This fixes a bug in the OpenTLD project:
https://github.com/zk00006/OpenTLD/issues/22

I'm using OpenCV 2.4.1 on Mac OS X.