VPP-534 Remove references to DTOs created in JVpp

JVM objects allocated from a native thread need to be freed,
they are not subject to GC.

Change-Id: If1e140d2ceaec93631735ae7665f45db5aacf7cf
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>

vpp-api/java/jvpp-registry/jvpp_registry.c

@@ -157,6 +157,7 @@ static void vl_api_control_ping_reply_t_handler(
 
             (*env)->CallVoidMethod(env, rm->registryObject, callbackMethod,
                     dto);
+            (*env)->DeleteLocalRef(env, dto);
         }
     }