Fix FTBFS with GCC6

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit 6ba476df60)

bibletime/frontend/mainindex/cmainindex.cpp

@@ -228,7 +228,7 @@ void CMainIndex::slotExecuted( TQListViewItem* i ) {
 /** Reimplementation. Returns the drag object for the current selection. */
 TQDragObject* CMainIndex::dragObject() {
 	if (!m_itemsMovable) {
-		return false;
+		return 0;
 	}
 
 	CDragDropMgr::ItemList dndItems;