bulletproof against num thread children getting to 0 r=jefft

mailnews/db/msgdb/src/nsMsgThread.cpp

@@ -830,6 +830,8 @@ nsresult nsMsgThread::ChangeChildCount(PRInt32 delta)
 	childCount += delta;
 
 	NS_ASSERTION((PRInt32) childCount >= 0, "child count gone to 0 or below");
+	if ((PRInt32) childCount <= 0)	// force child count to > 0
+		childCount = 1;
 
 	ret = m_mdbDB->UInt32ToRowCellColumn(m_metaRow, m_mdbDB->m_threadChildrenColumnToken, childCount);
 	m_numChildren = childCount;
@@ -867,6 +869,9 @@ nsresult nsMsgThread::GetChildHdrForKey(nsMsgKey desiredKey, nsIMsgDBHdr **resul
 
 	GetNumChildren(&numChildren);
 
+	if ((PRInt32) numChildren < 0)
+		numChildren = 0;
+
 	for (childIndex = 0; childIndex < numChildren; childIndex++)
 	{
 		rv = GetChildHdrAt(childIndex, result);