Fix a fatal in the "Projects" curtain extension when a project edge connects an object to a non-project

Summary:
Ref T13429. It's currently possible to write "TYPE_EDGE" relationships for the "object has project" edge to PHIDs which may not actually be projects. Today, this fatals.

As a first step, unfatal it. T13429 discusses general improvements and greater context.

Test Plan:
Used "maniphest.edit" to write a "project" edge to a user PHID, viewed the task in the UI. Previously it fataled; now it renders unusually (the object is "tagged" with a user) but faithfully reflects database state.

{F6957606}

Maniphest Tasks: T13429

Differential Revision: https://secure.phabricator.com/D20860

src/applications/project/engine/PhabricatorBoardLayoutEngine.php

@@ -301,6 +301,12 @@ final class PhabricatorBoardLayoutEngine extends Phobject {
       ->execute();
     $boards = mpull($boards, null, 'getPHID');
 
+    foreach ($boards as $key => $board) {
+      if (!($board instanceof PhabricatorWorkboardInterface)) {
+        unset($boards[$key]);
+      }
+    }
+
     if (!$this->fetchAllBoards) {
       foreach ($boards as $key => $board) {
         if (!$board->getHasWorkboard()) {