fix TurnArray: isEmpty must be synchronized

Author: lecousin

net.lecousin.core/src/main/java/net/lecousin/framework/collections/TurnArray.java

@@ -135,7 +135,7 @@ public synchronized T removeLast() {
 	}
 
 	@Override
-	public boolean isEmpty() {
+	public synchronized boolean isEmpty() {
 		return start == end;
 	}