gpcf

The server crashes repeatedly with the following traceback: ``` 2017-12-04 12:14:22: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod '' in callback node_on_receive_fields(): /var/games/minetest-server/.minetest/mods/digtron/util.lua:175: bad argument #1 to 'pairs'...

The building node can't place seeds to plant plants. This would be a cool addition, so the digtron could be used for agricultural purposes.