validate: media-check: fix double unref in error code path

Writer would get unrefed twice when it could not parse the file.

https://bugzilla.gnome.org/show_bug.cgi?id=750606

validate/tools/gst-validate-media-check.c

@@ -111,7 +111,6 @@ main (int argc, gchar ** argv)
 
     if (reference == NULL) {
       g_print ("Could not parse file: %s", expected_file);
-      gst_object_unref (writer);
       ret = 1;
       goto out;
     }