Add num docs sanity value to IndexManager, a=chris

diff --git a/src/library/IndexManager.php b/src/library/IndexManager.php
index 761f78374..d0f5f1f13 100644
--- a/src/library/IndexManager.php
+++ b/src/library/IndexManager.php
@@ -210,7 +210,7 @@ class IndexManager implements CrawlConstants
                             if ($add_flag) {
                                 $out_info[$record[0]] = $record;
                             } else {
-                                $total -= $record[3];
+                                $total = max(0, $total - $record[3]);
                             }
                         }
                         IndexManager::$dictionary[$id][$hash][$shift