Sniffers can dig into blocks outside the world border

The bug

Sniffers can dig into blocks outside the world border.

Reproduce steps

1. Create a Superflat world.

2. Use the following command:

   /worldborder set 4

   /worldborder set 4

3. Summon a sniffer.

4. Found the sniffer can dig into the blocks outside the world border.