Fix private tensor disclosure (#2434)

private tensors aren't meant to be accessible from a remote client,
however, execute_command was getting any object using his id, this fix
get the object using the get_obj method that doesn't return private
tensors

syft/workers/base.py

@@ -359,7 +359,9 @@ def execute_command(self, message: tuple) -> "pointers.PointerTensor":
         # Handle methods
         if _self is not None:
             if type(_self) == int:
-                _self = self._objects[_self]
+                _self = self.get_obj(_self)
+                if _self is None:
+                    return
             if type(_self) == str and _self == "self":
                 _self = self
             if sy.torch.is_inplace_method(command_name):