data: fix DNode double free

Calling free twice on a dnode is failing with libyang-python, when it is
supported with libyang. That is because cdata is set to None in the free
function. Set it to ffi.NULL so that libyang can see the NULL pointer.

Fixes: #84
Signed-off-by: Stefan Gula <steweg@gmail.com>
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>

Author: samuel-gauthier

libyang/data.py

@@ -881,7 +881,7 @@ def free(self, with_siblings: bool = True) -> None:
             else:
                 self.free_internal(with_siblings)
         finally:
-            self.cdata = None
+            self.cdata = ffi.NULL
 
     def __repr__(self):
         cls = self.__class__

tests/test_data.py

@@ -876,3 +876,8 @@ def test_add_defaults(self):
         node = dnode.find_path("/yolo-system:conf/speed")
         self.assertIsInstance(node, DLeaf)
         self.assertEqual(node.value(), 4321)
+
+    def test_dnode_double_free(self):
+        dnode = self.ctx.parse_data_mem(self.JSON_CONFIG, "json", validate_present=True)
+        dnode.free()
+        dnode.free()