Better sanity check handling WordITerator, position list decode 3

diff --git a/src/library/index_bundle_iterators/WordIterator.php b/src/library/index_bundle_iterators/WordIterator.php
index 661e880d8..05b4c2904 100644
--- a/src/library/index_bundle_iterators/WordIterator.php
+++ b/src/library/index_bundle_iterators/WordIterator.php
@@ -487,7 +487,7 @@ class WordIterator extends IndexBundleIterator
             IndexDocumentBundle::POSITIONS_FILENAME;
         $fh = (file_exists($positions_filename)) ?
             fopen($positions_filename, "r") : false;
-        $file_size = file_size($positions_filename);
+        $file_size = filesize($positions_filename);
         $number_of_partitions = $this->total_number_of_partitions;
         $num_doc_keys = $doc_map_tools->countTableEntries($doc_map_filename);
         $is_ascending = ($this->direction == self::ASCENDING);