common.c: do not destroy nn_queue if parent object is NULL (segfault)
common.c: do not destroy nn_queue if parent object is NULL (segfault)

TidyHuang/sdn_sensor,TidyHuang/sdn_sensor,megahall/sdn_sensor,TidyHuang/sdn_sensor,megahall/sdn_sensor,megahall/sdn_sensor,TidyHuang/sdn_sensor,megahall/sdn_sensor,megahall/sdn_sensor,megahall/sdn_sensor,megahall/sdn_sensor,megahall/sdn_sensor,TidyHuang/sdn_sensor,TidyHuang/sdn_sensor,TidyHuang/sdn_sensor,TidyHuang/sdn_sensor