CChannelList: add missing compare for existant header object.

Origin commit data
------------------
Commit: 86c18141dd
Author: Thilo Graf <dbt@novatux.de>
Date: 2017-05-06 (Sat, 06 May 2017)

src/gui/channellist.cpp

@@ -2158,13 +2158,17 @@ void CChannelList::paintHead()
 
 CComponentsHeader* CChannelList::getHeaderObject()
 {
+	if (header)
 		return header;
+	return NULL;
 }
 
 void CChannelList::ResetModules()
 {
+	if (header){
 		delete header;
 		header = NULL;
+	}
 	if(dline){
 		delete dline;
 		dline = NULL;