Check for digiline message field type

[Emojigit]

This PR fixes a fatal error: if a malformed digiline table is passed into a node detector, the server may crash because of a mismatched data type.

For example, this weird backtrace happened because technic is sending "malformed" tables:

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod '??' in callback node_on_timer(): $HOME/.minetest/mods/mesecons/mesecons_detector/init.lua:63: bad argument #2 to 'set_string' (string expected, got table)
ERROR[Main]: stack traceback:
ERROR[Main]:       [C]: in function 'set_string'
ERROR[Main]:       $HOME/.minetest/mods/mesecons/mesecons_detector/init.lua:63: in function 'action'
ERROR[Main]:       $HOME/.minetest/mods/digicontrol/override.lua:45: in function 'transmit'
ERROR[Main]:       $HOME/.minetest/mods/digicontrol/override.lua:78: in function 'receptor_send'
ERROR[Main]:       ...test/mods/technic/technic/machines/switching_station.lua:112: in function <...test/mods/technic/technic/machines/switching_station.lua:87>

[Niklp09]

@Emojigit bump.