Better sanity check handling WordITerator, position list decode

diff --git a/src/library/index_bundle_iterators/WordIterator.php b/src/library/index_bundle_iterators/WordIterator.php
index 81a710799..6a9bbb5cb 100644
--- a/src/library/index_bundle_iterators/WordIterator.php
+++ b/src/library/index_bundle_iterators/WordIterator.php
@@ -497,10 +497,13 @@ class WordIterator extends IndexBundleIterator
         foreach ($postings as $posting) {
             $posting[self::GENERATION] = $partition;
             if ($posting['POSITIONS_LEN'] > 0 && !empty($fh)) {
-                fseek($fh, $posting['POSITIONS_OFFSET']);
-                $encoded_positions = fread($fh, $posting['POSITIONS_LEN']);
-                $posting[self::POSITION_LIST] = L\decodePositionList(
-                    $encoded_positions, $posting['FREQUENCY']);
+                if (fseek($fh, $posting['POSITIONS_OFFSET']) >= 0) {
+                    $encoded_positions = fread($fh, $posting['POSITIONS_LEN']);
+                    $posting[self::POSITION_LIST] = L\decodePositionList(
+                        $encoded_positions, $posting['FREQUENCY']);
+                } else {
+                    $posting[self::POSITION_LIST] = [];
+                }
             } else {
                 $posting[self::POSITION_LIST] = [];
             }