don't allow thread unread counts to be negative r=naving, sr=mscott fixes one case of 64476

mailnews/db/msgdb/src/nsMsgThread.cpp

@@ -94,6 +94,8 @@ nsresult nsMsgThread::InitCachedValues()
 //    NS_ASSERTION(m_numChildren <= rowCount, "num children wrong - fixing");
     if (m_numChildren > rowCount)
       ChangeChildCount((PRInt32) rowCount - (PRInt32) m_numChildren);
+    if ((PRInt32) m_numUnreadChildren < 0)
+      ChangeUnreadChildCount(- (PRInt32) m_numUnreadChildren);
 		if (NS_SUCCEEDED(err))
 			m_cachedValuesInitialized = PR_TRUE;
 	}
@@ -963,7 +965,13 @@ nsresult nsMsgThread::ChangeUnreadChildCount(PRInt32 delta)
   PRUint32 childCount = 0;
   m_mdbDB->RowCellColumnToUInt32(m_metaRow, m_mdbDB->m_threadUnreadChildrenColumnToken, childCount);
   childCount += delta;
-
+  if ((PRInt32) childCount < 0)
+  {
+#ifdef DEBUG_bienvenu
+    NS_ASSERTION(PR_FALSE, "negative unread child count");
+#endif
+    childCount = 0;
+  }
   ret = m_mdbDB->UInt32ToRowCellColumn(m_metaRow, m_mdbDB->m_threadUnreadChildrenColumnToken, childCount);
   m_numUnreadChildren = childCount;
   return ret;