BUG Rewrite hashlinks failing on empty a tags

Check for presence of href in _a_ tags before trying to .match them to
prevent null pointer errors.

Some external JS libs (eg tag-it) use <ul><a> structures which get matched
and break

(fixes regression introduced by ff39f9ad38)

admin/javascript/LeftAndMain.js

@@ -951,6 +951,7 @@ jQuery.noConflict();
 			 */
 			rewriteHashlinks: function() {
 				$(this).find('ul a').each(function() {
+					if (!$(this).attr('href')) return;
 					var matches = $(this).attr('href').match(/#.*/);
 					if(!matches) return;
 					$(this).attr('href', document.location.href.replace(/#.*/, '') + matches[0]);