Fix leaked globals.

```
00:40:25: ERROR[ServerThread]: Assignment to undeclared global "node" inside a function at ...minetest\bin\..\games\minetest_game\mods\bucket\init.lua:138.
00:40:25: ERROR[ServerThread]: Assignment to undeclared global "liquiddef" inside a function at ...minetest\bin\..\games\minetest_game\mods\bucket\init.lua:139.
```