highgui: fix segfault on CvCapture_GStreamer::open
highgui: fix segfault on CvCapture_GStreamer::open

when compiled with GStreamer, open (of a file) segfaults.

Fix was suggested by Bostjan Vesnicer.

opencv/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,apavlenko/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv,opencv/opencv,apavlenko/opencv,opencv/opencv,opencv/opencv