Fix matter registry gets invalidated on second join in singleplayer

src/main/java/ru/dbotthepony/mc/otm/matter/MatterRegistry.java

@@ -452,8 +452,15 @@ public class MatterRegistry {
 		}
 	}
 
+	private static boolean started_once = false;
+
 	@SubscribeEvent
 	public static void serverStarted(FMLServerStartedEvent event) {
+		if (started_once)
+			return;
+
+		started_once = true;
+
 		ITEMS.clear();
 		usages.clear();
 		results.clear();