Bugfix: cores number could be 0

src/com/sheepit/client/standalone/Worker.java

@@ -183,7 +183,7 @@ public class Worker {
 			}
 		}
 		
-		if (nb_cores < -1) {
+		if (nb_cores < -1 || nb_cores == 0) { // -1 is the default
 			System.err.println("Error: use-number-core should be a greater than zero");
 			return;
 		}