More type checking in buildInvertedIndexPartition, a=chris

diff --git a/src/library/IndexDocumentBundle.php b/src/library/IndexDocumentBundle.php
index 5fbfa2517..97790453e 100644
--- a/src/library/IndexDocumentBundle.php
+++ b/src/library/IndexDocumentBundle.php
@@ -621,7 +621,7 @@ class IndexDocumentBundle implements CrawlConstants
             $non_aux_doc_cnt++;
             if (!empty($url_info[$doc_field])) {
                 $site = $this->getSummary($url_info[$doc_field], $partition);
-                if (empty($site)) {
+                if (empty($site) || !is_array($site)) {
                     continue;
                 }
             }