TreeNode: fixed runtime asan warning (#2920)

imgui_widgets.cpp:6923:52: runtime error: shift exponent -1 is negative

imgui_widgets.cpp

@@ -6914,7 +6914,7 @@ void ImGui::TreeNodeDrawLineToChildNode(const ImVec2& target_pos)
 {
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = g.CurrentWindow;
-    if ((window->DC.TreeHasStackDataDepthMask & (1 << (window->DC.TreeDepth - 1))) == 0)
+    if (window->DC.TreeDepth == 0 || (window->DC.TreeHasStackDataDepthMask & (1 << (window->DC.TreeDepth - 1))) == 0)
         return;
 
     ImGuiTreeNodeStackData* parent_data = &g.TreeNodeStack.Data[g.TreeNodeStack.Size - 1];