Bug 1280 - clutter_score_append_at_marker lead to segmentation

	Bug 1280 - clutter_score_append_at_marker lead to segmentation
	fault in trunk and incorrect appearance in clutter-0.8

	* clutter/clutter-score.c (start_children_entries): Check whether
	the child timeline is actually attached at a marker before
	comparing whether the marker's name matches the marker
	reached. This fixes a crash that happens when a marker is reached
	on a timeline that also has child timelines attached at the
	end. Thanks to zhangwei for spotting.