cv::stereoRectify: fix segfault in case of empty distCoeffs
cv::stereoRectify: fix segfault in case of empty distCoeffs

cvStereoRectify assumes that NULL is provided in case of no distCoeffs

opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv,opencv/opencv