Free up unused field var's before doing addPartitionPostingsDictionary, a=chris

diff --git a/src/library/IndexDocumentBundle.php b/src/library/IndexDocumentBundle.php
index 7b2183c08..5fbfa2517 100644
--- a/src/library/IndexDocumentBundle.php
+++ b/src/library/IndexDocumentBundle.php
@@ -430,6 +430,8 @@ class IndexDocumentBundle implements CrawlConstants
         }
         $base_folder = $this->getPartitionBaseFolder($partition);
         $postings_tools = $this->postings_tools;
+        unset($this->postings, $this->doc_map, $this->positions,
+            $this->last_entries);
         $last_entries_tools = $this->last_entries_tools;
         $dictionary = $this->dictionary;
         $postings_filename = $base_folder . "/" . self::POSTINGS_FILENAME;