Fixes a rare notice in index dictionary, a=chris

diff --git a/lib/index_dictionary.php b/lib/index_dictionary.php
index 4c713b9a7..86303fbb0 100644
--- a/lib/index_dictionary.php
+++ b/lib/index_dictionary.php
@@ -580,7 +580,9 @@ class IndexDictionary implements CrawlConstants
         if($prefix_info == IndexShard::BLANK) {
             return false;
         }
-        list(, $offset, $high) = unpack("N*", $prefix_info);
+        $prefix_tmp = unpack("N*", $prefix_info);
+        if(!$prefix_tmp[2]) {return false; }
+        list(, $offset, $high) = $prefix_tmp;
         $high--;
         $start = self::PREFIX_HEADER_SIZE  + $offset;
         $low = 0;