Fix convert_data crash when craft data is missing

Crash occurs when no data for "craft" is available in the old data, fixed with a additional check.

api.lua

@@ -51,6 +51,7 @@ local function convert_data()
 			local ret = {}
 
 			local count = 0
+			if data[from] then
 				for modname, items in pairs(data[from]) do
 					for itemname, value in pairs(items) do
 						itemname = modname .. ":" .. itemname
@@ -59,6 +60,7 @@ local function convert_data()
 						count = count + value
 					end
 				end
+			end
 
 			ret.__total = count
 			data[from] = nil