Resolve "PHP Warnungen im WIKI"

Closes #3967

Merge request studip/studip!2832

app/controllers/course/wiki.php

@@ -389,7 +389,7 @@ class Course_WikiController extends AuthenticatedController
             $this->redirect($this->editURL($page));
             return;
         }
-        if (!$page->isEditable()) {
+        if ($page->isNew() || !$page->isEditable()) {
             throw new AccessDeniedException();
         }
         Navigation::activateItem('/course/wiki/start');

app/views/course/wiki/newpages.php

@@ -69,7 +69,9 @@
                         break;
                     }
                 }
-                $oldcontent = $oldversion->content;
+                if ($oldversion) {
+                    $oldcontent = $oldversion->content;
+                }
                 $oldcontent = strip_tags(wikiReady($oldcontent));
                 $content = strip_tags(wikiReady($page->content));