6417649: -interval=0 is accepted and jconsole doesn't update window content at all

Reviewed-by: alanb, jbachorik

jdk/src/share/classes/sun/tools/jconsole/JConsole.java

@@ -858,6 +858,10 @@ public class JConsole extends JFrame
                 try {
                     updateInterval = Integer.parseInt(arg.substring(10)) *
                         1000;
+                    if (updateInterval <= 0) {
+                        usage();
+                        return;
+                    }
                 } catch (NumberFormatException ex) {
                     usage();
                     return;