Fix Notice IndexShard.php, a=chris

diff --git a/src/library/IndexShard.php b/src/library/IndexShard.php
index 514efa1f5..ff73fed32 100644
--- a/src/library/IndexShard.php
+++ b/src/library/IndexShard.php
@@ -557,7 +557,7 @@ class IndexShard extends PersistentStructure implements CrawlConstants
             $prefix = (ord($word_id[0]) << 8) + ord($word_id[1]);
             $prefix_info = $this->getShardSubstring(
                 self::HEADER_LENGTH + 8 * $prefix, 8);
-            if ($prefix_info == self::BLANK) {
+            if ($prefix_info == self::BLANK || !isset($prefix_info[2])) {
                 return false;
             }
             list(,$offset, $high) = unpack("N*", $prefix_info);