Avoid error if creation date calendar is empty.

Avoid error on picture_modify.php if creation date is not correct.

git-svn-id: http://piwigo.org/svn/trunk@5283 68402e56-0260-453c-a942-63ccdbb3a9ee

include/functions_calendar.inc.php

@@ -292,6 +292,7 @@ WHERE id IN (' . implode(',',$page['items']) .')';
 
 function array_to_int($array)
 {
+  $result = array();
   foreach ($array as $k => $v)
   {
     if ((int)$k==$k)