Fix JS error when un-summarizing old messages.

`Cannot read property 'flags' of undefined` in the
_.each callback in expand_summary_row.

Messages loaded when you scroll up in a narrow are not added to
all_msg_list. Because the user just clicked the message, we know
the message is in current_msg_list, so use that instead.

(imported from commit e76449a2a2748b96f69a2ab05d288b708d9e3ac0)

static/js/ui.js

@@ -791,7 +791,7 @@ exports.collapse = function (row) {
 exports.expand_summary_row = function (row) {
     var message_ids = row.attr('data-messages').split(' ');
     var messages = _.map(message_ids, function (id) {
-        return all_msg_list.get(id);
+        return current_msg_list.get(id);
     });
 
     _.each(messages, function (msg){