Issue #3989 - Restoring _selector to null on failure

Signed-off-by: Joakim Erdfelt <joakim.erdfelt@gmail.com>

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

@@ -510,6 +510,7 @@ public class ManagedSelector extends ContainerLifeCycle implements Dumpable
                     LOG.warn(x.toString());
                     LOG.debug(x);
                 }
+                _selector = null;
                 IO.close(selector);
             }
             return false;