Fix security bug in thumb generation, a=chris

diff --git a/src/models/GroupModel.php b/src/models/GroupModel.php
index 1d01527a8..299e0e0db 100644
--- a/src/models/GroupModel.php
+++ b/src/models/GroupModel.php
@@ -2268,7 +2268,7 @@ class GroupModel extends Model implements MediaConstants
      * @param string $mime_type the mime type of the file
      * @return bool whether a thumb was made or not
      */
-    function makeThumbStripExif ($file_name, $folder, $thumb_folder,
+    function makeThumbStripExif($file_name, $folder, $thumb_folder,
         $mime_type = "")
     {
         if ($mime_type == "") {
@@ -2428,6 +2428,9 @@ class GroupModel extends Model implements MediaConstants
             $resource['is_compressed'] = false;
             $resource['is_writable'] = false;
             $resource['media_type'] = false;
+            if (!file_exists($pre_resource)) {
+                continue;
+            }
             if (is_dir($pre_resource)) {
                 $resource['is_dir'] = true;
             } else {
@@ -2448,7 +2451,7 @@ class GroupModel extends Model implements MediaConstants
             } else if (!$resource['is_dir'] &&
                 time() < $time + C\PAGE_TIMEOUT/2) {
                 $resource['has_thumb'] =
-                    $this->makeThumbStripExif ($name, $folder, $thumb_folder);
+                    $this->makeThumbStripExif $name, $folder, $thumb_folder);
             }
             $resources[] = $resource;
         }