Fix Pipeline NPE or sth with multi

* followings are now throwing JedisDataException: it was uncontrolled or controlled by Redis itself
** exec without multi
** discard without multi
** multi within multi
* updates unit test

actually Redis returns ERR and we can pick, but Pipeline + multi has some complex sequence
so it can just throw NPE without ERR

src/main/java/redis/clients/jedis/Pipeline.java

@@ -104,12 +104,18 @@ public class Pipeline extends MultiKeyPipelineBase {
     }
 
     public Response<String> discard() {
+	if (currentMulti == null)
+	    throw new JedisDataException("DISCARD without MULTI");
+
 	client.discard();
 	currentMulti = null;
 	return getResponse(BuilderFactory.STRING);
     }
 
     public Response<List<Object>> exec() {
+	if (currentMulti == null)
+	    throw new JedisDataException("EXEC without MULTI");
+
 	client.exec();
 	Response<List<Object>> response = super.getResponse(currentMulti);
 	currentMulti.setResponseDependency(response);
@@ -118,6 +124,9 @@ public class Pipeline extends MultiKeyPipelineBase {
     }
 
     public Response<String> multi() {
+	if (currentMulti != null)
+	    throw new JedisDataException("MULTI calls can not be nested");
+
 	client.multi();
 	Response<String> response = getResponse(BuilderFactory.STRING); // Expecting
 									// OK