Fix UI breakage that prevents switching collections

I'll add a test later once I figure out how to reproduce this, but I
think this should fix the "id is undefined" errors.

chrome/content/zotero/xpcom/libraryTreeView.js

@@ -130,7 +130,7 @@ Zotero.LibraryTreeView.prototype = {
 	 * Restore a scroll position returned from _saveScrollPosition()
 	 */
 	_rememberScrollPosition: function (scrollPosition) {
-		if (!scrollPosition) {
+		if (!scrollPosition || !scrollPosition.id) {
 			return;
 		}
 		var row = this.getRowIndexByID(scrollPosition.id);