api.lua:1242: attempt to index a nil value

[cheapie]

api.lua's sign post conversion LBM crashes when encountering a "signs:sign_post" node with certain param2 values. Most likely any param2 value where param2 % 8 >= 4 will do it, but I only tested with 5.

Example output:

ERROR[Main]: ServerError: AsyncErr: Lua: Runtime error from mod 'signs_lib' in callback LuaLBM::trigger(): ...mt/bin/../games/dreambuilder_game/mods/signs_lib/api.lua:1242: attempt to index a nil value
ERROR[Main]: stack traceback:
ERROR[Main]: 	...mt/bin/../games/dreambuilder_game/mods/signs_lib/api.lua:1242: in function 'func'
ERROR[Main]: 	/home/noah/mt/bin/../builtin/profiler/instrumentation.lua:124: in function 'action'
ERROR[Main]: 	/home/noah/mt/bin/../builtin/game/misc.lua:320: in function </home/noah/mt/bin/../builtin/game/misc.lua:304>

This was initially discovered because it was triggered by a corrupted map, but it is possible (but complex) to trigger it without corruption:

1. Using another mod, register signs:sign_post as a node, preferably with a paramtype2 value of facedir to make the next step easier.
2. Place a signs:sign_post node and set its param2 to 5. If paramtype2 is facedir then a screwdriver can probably be used for this.
3. Reload the mapblock (restarting the server works). The server will crash when the mapblock is loaded.

I suspect other methods of obtaining a signs:sign_post (some old now-removed mod that had it, an old version, or whatever) would have the same effect.

As a side note, does this LBM really need to use run_at_every_load = true? It seems like a textbook example of using false there.