Fixes #9947 - check that each selector inside of _selectors isn't nul…

…l before calculating #totalKeys (#9962)

Check that selector isn't null before calculating totalKeys.

jetty-io/src/main/java/org/eclipse/jetty/io/SelectorManager.java

@@ -148,14 +148,14 @@ protected void execute(Runnable task)
     public int getTotalKeys()
     {
         int keys = 0;
-        for (final ManagedSelector selector : _selectors)
+        for (ManagedSelector selector : _selectors)
         {
-            keys += selector.getTotalKeys();
+            if (selector != null)
+                keys += selector.getTotalKeys();
         }
         return keys;
     }
 
-
     /**
      * @return the number of selectors in use
      */