merge r3656 from branch 2.0 to trunk

feature 953: avoid fatal error when delete a photo (added with pwg.images.add
API) and the file doesn't not exist on the filesystem.

git-svn-id: http://piwigo.org/svn/trunk@3657 68402e56-0260-453c-a942-63ccdbb3a9ee

admin/include/functions.php

@@ -171,7 +171,7 @@ SELECT
 
       foreach (array($file_path, $thumbnail_path, $high_path) as $path)
       {
-        if (isset($path) and !unlink($path))
+        if (isset($path) and is_file($path) and !unlink($path))
         {
           die('"'.$path.'" cannot be removed');
         }