Fix Minetest blaming the wrong mod for errors (#12241)

Covers the case where mods insert their callbacks manually into "minetest.registered_<callbacks>" (often to achieve a particular order of execution).

builtin/game/register.lua

@@ -403,8 +403,14 @@ function core.override_item(name, redefinition)
 	register_item_raw(item)
 end
 
-
-core.callback_origins = {}
+do
+	local default = {mod = "??", name = "??"}
+	core.callback_origins = setmetatable({}, {
+		__index = function()
+			return default
+		end
+	})
+end
 
 function core.run_callbacks(callbacks, mode, ...)
 	assert(type(callbacks) == "table")
@@ -419,9 +425,7 @@ function core.run_callbacks(callbacks, mode, ...)
 	local ret = nil
 	for i = 1, cb_len do
 		local origin = core.callback_origins[callbacks[i]]
-		if origin then
-			core.set_last_run_mod(origin.mod)
-		end
+		core.set_last_run_mod(origin.mod)
 		local cb_ret = callbacks[i](...)
 
 		if mode == 0 and i == 1 then