Fix disposing wxComboBox with data objects

Under wxOSX the order of calls to wxItemContainer::Clear()
and wxTextEntry::Clear() matters.
Number of data objects to delete in wxItemContainer::Clear()
is obtained by call to wxComboBox::Count(). Counter of items
in wxComboBox is reset by wxTextEntry()::Clear()
so this method has to be called after wxItemContainer::Clear().

Closes #18768.

include/wx/combobox.h

@@ -35,8 +35,8 @@ public:
     // override these methods to disambiguate between two base classes versions
     virtual void Clear() wxOVERRIDE
     {
-        wxTextEntry::Clear();
         wxItemContainer::Clear();
+        wxTextEntry::Clear();
     }
 
     // IsEmpty() is ambiguous because we inherit it from both wxItemContainer