highgui: fix segfault on CvCapture_GStreamer::open

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

Fix was suggested by Bostjan Vesnicer.

modules/highgui/src/cap_gstreamer.cpp

@@ -399,12 +399,12 @@ bool CvCapture_GStreamer::open( int type, const char* filename )
 
     gst_app_sink_set_max_buffers (GST_APP_SINK(sink), 1);
     gst_app_sink_set_drop (GST_APP_SINK(sink), stream);
-
-    gst_app_sink_set_caps(GST_APP_SINK(sink), gst_caps_new_simple("video/x-raw-rgb",
+    caps = gst_caps_new_simple("video/x-raw-rgb",
                                "red_mask",   G_TYPE_INT, 0x0000FF,
                                "green_mask", G_TYPE_INT, 0x00FF00,
                                "blue_mask",  G_TYPE_INT, 0xFF0000,
-                                                                  NULL));
+                               NULL);
+    gst_app_sink_set_caps(GST_APP_SINK(sink), caps);
     gst_caps_unref(caps);
 
     if(gst_element_set_state(GST_ELEMENT(pipeline), GST_STATE_READY) ==