fix filter after the fact not to have cumulative matches, r/sr=sspitzer, 168551

mailnews/base/search/src/nsMsgFilterService.cpp

@@ -471,6 +471,7 @@ NS_IMETHODIMP nsMsgFilterAfterTheFact::OnSearchDone(nsresult status)
 NS_IMETHODIMP nsMsgFilterAfterTheFact::OnNewSearch()
 {
   m_searchHits.RemoveAll();
+  m_searchHitHdrs->Clear();
   return NS_OK;
 }