mounts/ostree.deployment: initialize fields

Initialize `mountpoint` and `check` fields. In case of an error
in `open` not having `mountpoint` or `check` initialized will
cause another exception. So this is mostly important in case of
error, but it is the right thing anyway.

mounts/org.osbuild.ostree.deployment

@@ -61,6 +61,12 @@ SCHEMA_2 = """
 
 class OSTreeDeploymentMount(mounts.MountService):
 
+    def __init__(self, args):
+        super().__init__(args)
+
+        self.mountpoint = None
+        self.check = False
+
     @staticmethod
     def bind_mount(source, target):
         subprocess.run([