Initialize all pointers of CvCapture_GStreamer correctly

Opening a non-existing video file with ``cv2.videoCapture()`` (in Python) and releasing the video object will result in a segfault caused by uninitialized memory. The 'color' pointer (as well as a few other pointers) was not set to NULL, and therefore uses uninitialized memory which caused the segfault on ``CvCapture_GStreamer::close()``.