White space formatting and make sure we don't hit null if the parent isn't visible.

src/MarkerClusterGroup.js

@@ -390,14 +390,13 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 		this._map = null;
 	},
 
-   getVisibleParent: function(marker){
-      var vMarker = marker;
-      // TODO: Is this safe? No extra check needed to prevent infinite loop?
-      while(!vMarker._icon) {
-         vMarker = vMarker.__parent;
-      }
-      return vMarker;
-   },
+	getVisibleParent: function(marker){
+		var vMarker = marker;
+		while(vMarker !== null && !vMarker._icon) {
+			vMarker = vMarker.__parent;
+		}
+		return vMarker;
+	},
 
 	//Remove the given object from the given array
 	_arraySplice: function (anArray, obj) {