SLING-5006: set executorService to null on deactivate

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1712046 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/java/org/apache/sling/serviceusermapping/impl/ServiceUserMapperImpl.java b/src/main/java/org/apache/sling/serviceusermapping/impl/ServiceUserMapperImpl.java
index f4f7bd9..6b545f1 100644
--- a/src/main/java/org/apache/sling/serviceusermapping/impl/ServiceUserMapperImpl.java
+++ b/src/main/java/org/apache/sling/serviceusermapping/impl/ServiceUserMapperImpl.java
@@ -157,6 +157,7 @@
         this.executeServiceRegistrationsAsync(registrationSet);
         if (executorService != null) {
             executorService.shutdown();
+            executorService = null;
         }
     }