Fixed assertion that could appear under wxGTK if wxComboCtrl drop-down button was clicked too quickly (backported from trunk)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@59689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/combocmn.cpp

@@ -1789,7 +1789,10 @@ void wxComboCtrlBase::OnButtonClick()
 {
     // Derived classes can override this method for totally custom
     // popup action
-    ShowPopup();
+    if ( !IsPopupWindowState(Visible) )
+        ShowPopup();
+    else
+        HidePopup();
 }
 
 void wxComboCtrlBase::ShowPopup()