FailureDetector: check that states is not null before use

When a node is part of a cluster but is down (like in the situation where
a cluster is taken down and up again but not all nodes are up). There is
no application_state information for that node.

This patch check that the information exists before using it to prevent
null pointer exception.

After this patch, a call to nodetool gossipinfo would return the
available information without failing.

See scylladb/scylla#3330

Signed-off-by: Amnon Heiman <amnon@scylladb.com>
Message-Id: <20180329115345.29357-1-amnon@scylladb.com>
(cherry picked from commit 4e4589ba6f)

src/main/java/org/apache/cassandra/gms/FailureDetector.java

@@ -105,10 +105,12 @@ public class FailureDetector extends APIMBean implements FailureDetectorMBean {
             ep.setAliave(obj.getBoolean("is_alive"));
             ep.setUpdateTimestamp(obj.getJsonNumber("update_time").longValue());
             JsonArray states = obj.getJsonArray("application_state");
+            if (states != null) {
                 for (int j = 0; j < states.size(); j++) {
                     JsonObject state = states.getJsonObject(j);
                     ep.addApplicationState(state.getInt("application_state"), state.getString("value"));
                 }
+            }
             res.put(obj.getString("addrs"), ep);
         }
         return res;