clutter-score: Initialize ClutterScoreEntry::complete_id

When clutter_score_append_at_marker is called instead of
clutter_score_append the complete_id field of ClutterScoreEntry was
being left uninitialised. When the entry is eventually freed it would
sometimes try to disconnect an invalid signal id. This was causing
conformance test failures for me on GLES2.

clutter/clutter-score.c

@@ -990,6 +990,7 @@ clutter_score_append_at_marker (ClutterScore    *score,
   entry->marker = g_strdup (marker_name);
   entry->id = priv->last_id;
   entry->score = score;
+  entry->complete_id = 0;
 
   marker_reached_signal = g_strdup_printf ("marker-reached::%s", marker_name);
   entry->marker_id = g_signal_connect (entry->parent,