Tests: avoid NPE in JettySolrRunner lifeCycleStopped (#2337)

diff --git a/solr/test-framework/src/java/org/apache/solr/embedded/JettySolrRunner.java b/solr/test-framework/src/java/org/apache/solr/embedded/JettySolrRunner.java
index 8fce258..edb7694 100644
--- a/solr/test-framework/src/java/org/apache/solr/embedded/JettySolrRunner.java
+++ b/solr/test-framework/src/java/org/apache/solr/embedded/JettySolrRunner.java
@@ -383,7 +383,9 @@
 
             @Override
             public synchronized void lifeCycleStopped(LifeCycle arg0) {
-              coreContainerProvider.close();
+              if (coreContainerProvider != null) {
+                coreContainerProvider.close();
+              }
             }
 
             @Override