Decrement CyObject after automatic arg conversion in defnode wrapper

Cython/Compiler/Nodes.py

@@ -3671,6 +3671,9 @@ class DefNodeWrapper(FuncDefNode):
                     code.put_var_xdecref(entry)
                 else:
                     code.put_var_decref(entry)
+            elif entry.is_arg and entry.type.is_cyp_class:
+                # the conversion from PyObject to CyObject creates a new CyObject reference
+                code.put_cyxdecref(entry.cname)
 
         code.put_finish_refcount_context()
         if not self.return_type.is_void: