MDL-52054 forum: Prevent error when clicking move without a destination

mod/forum/discuss.php

@@ -325,7 +325,7 @@ if ($forum->type != 'single'
         if (!empty($forummenu)) {
             echo '<div class="movediscussionoption">';
             $select = new url_select($forummenu, '',
-                    array(''=>get_string("movethisdiscussionto", "forum")),
+                    array('/mod/forum/discuss.php?d=' . $discussion->id => get_string("movethisdiscussionto", "forum")),
                     'forummenu', get_string('move'));
             echo $OUTPUT->render($select);
             echo "</div>";

mod/forum/tests/behat/move_discussion.feature

@@ -37,3 +37,5 @@ Feature: A teacher can move discussions between forums
     When I set the field "jump" to "Test forum 2"
     And I press "Move"
     Then I should see "This discussion has been moved to 'Test forum 2'."
+    And I press "Move"
+    And I should see "Discussion 1"