fix another notice in unpackPostings, a=chris

diff --git a/src/library/IndexDocumentBundle.php b/src/library/IndexDocumentBundle.php
index 39110aebf..4ef638ba6 100644
--- a/src/library/IndexDocumentBundle.php
+++ b/src/library/IndexDocumentBundle.php
@@ -1417,6 +1417,9 @@ class IndexDocumentBundle implements CrawlConstants
         $positions_offset = 0;
         $len_posting_strings = strlen($postings_string);
         for ($i = 0; $i < $num_items; $i++) {
+            if (!isset($postings_string[$current_pos])) {
+                crawlLog("Posting decode error");
+            }
             $int_info = ord($postings_string[$current_pos]);
             $current_pos++;
             $len_unpack_info = $unpack_len_map[$int_info];