Fix CppClassType is_subclass crashing with types without base classes

Cython/Compiler/PyrexTypes.py

@@ -3795,6 +3795,7 @@ class CppClassType(CType):
     def is_subclass(self, other_type):
         if self.same_as_resolved_type(other_type):
             return 1
+        if self.base_classes:
             for base_class in self.base_classes:
                 if base_class.is_subclass(other_type):
                     return 1