Be more defensive with global table initialization

When upgrading or downgrading the inventory_sync plugin it's possible
an different structure of the global.inventory_sync table is left behind
which could cause the code to crash when certain sub tables are not
present.

Be more defensive in the initialization of the table to make sure all
sub-tables are present on startup.

plugins/inventory_sync/module/inventory_sync.lua

@@ -29,15 +29,24 @@ inventory_sync.upload_inventory = upload_inventory
 inventory_sync.add_commands = add_commands
 inventory_sync.events = {}
 inventory_sync.events[clusterio_api.events.on_server_startup] = function(event)
-	-- set up global table
+	-- Set up global table
 	if not global.inventory_sync then
-		global.inventory_sync = {
-			download_start_tick = {},
-			saved_crafting_queue = {},
-			download_cache = {},
-			players_waiting_for_download = {},
-			players = {},
-		}
+		global.inventory_sync = {}
+	end
+	if not global.inventory_sync.download_start_tick then
+		global.inventory_sync.download_start_tick = {}
+	end
+	if not global.inventory_sync.saved_crafting_queue then
+		global.inventory_sync.saved_crafting_queue = {}
+	end
+	if not global.inventory_sync.download_cache then
+		global.inventory_sync.download_cache = {}
+	end
+	if not global.inventory_sync.players_waiting_for_download then
+		global.inventory_sync.players_waiting_for_download = {}
+	end
+	if not global.inventory_sync.players then
+		global.inventory_sync.players = {}
 	end
 end