6429289: (se) sun.nio.ch.SelectorImpl.processDeregisterQueue creates excessive garbage

Check if the cancelledKeys is empty or not before creating iterator

Reviewed-by: alanb

jdk/src/share/classes/sun/nio/ch/SelectorImpl.java

@@ -142,6 +142,7 @@ abstract class SelectorImpl
         // Precondition: Synchronized on this, keys, and selectedKeys
         Set cks = cancelledKeys();
         synchronized (cks) {
+            if (!cks.isEmpty()) {
                 Iterator i = cks.iterator();
                 while (i.hasNext()) {
                     SelectionKeyImpl ski = (SelectionKeyImpl)i.next();
@@ -158,6 +159,7 @@ abstract class SelectorImpl
                 }
             }
         }
+    }
 
     protected abstract void implDereg(SelectionKeyImpl ski) throws IOException;