fixed a bug in trigger::dbl_click()

impl_->set_expanded() was called unconditionally, even if the node had no children. This caused the node icon to change to the "expanded" icon if the node had an icon scheme, even when the node didn't have children.

source/gui/widgets/treebox.cpp

@@ -1952,7 +1952,7 @@ namespace nana
 					impl_->attr.tree_cont.for_each<item_locator&>(shape.first, nl);
 
 					auto const node = nl.node();
-					if (!node)
+					if (!node || !node->child)
 						return;
 
 					switch (nl.what())