Add num docs sanity value to IndexManager 2, a=chris

diff --git a/src/library/IndexManager.php b/src/library/IndexManager.php
index d0f5f1f13..c8dad3b5b 100644
--- a/src/library/IndexManager.php
+++ b/src/library/IndexManager.php
@@ -210,7 +210,9 @@ class IndexManager implements CrawlConstants
                             if ($add_flag) {
                                 $out_info[$record[0]] = $record;
                             } else {
-                                $total = max(0, $total - $record[3]);
+                                if($record[3] < $total) {
+                                    $total -= $record[3];
+                                }
                             }
                         }
                         IndexManager::$dictionary[$id][$hash][$shift