playback-test: avoid critical on exit

Only free vis_entries array when not null on exit.

https://bugzilla.gnome.org/show_bug.cgi?id=755201

tests/examples/playback/playback-test.c

@@ -3385,6 +3385,7 @@ reset_app (PlaybackApp * app)
   g_list_free (app->paths);
   g_list_foreach (app->sub_paths, (GFunc) g_free, NULL);
   g_list_free (app->sub_paths);
+  if (app->vis_entries)
     g_array_free (app->vis_entries, TRUE);
   g_print ("free pipeline\n");
   gst_object_unref (app->pipeline);