Fix zk::force_close and destructor to reset pointer
Fix zk::force_close and destructor to reset pointer

roselleebarle04/jubatus,jubatus/jubatus,kmaehashi/jubatus,jubatus/jubatus,gintenlabo/jubatus,Asuka52/jubatus,elkingtonmcb/jubatus,kmaehashi/jubatus,kmaehashi/jubatus_core,kmaehashi/jubatus_core,kmaehashi/jubatus_core,kumagi/jubatus_core,mathn/jubatus,kumagi/jubatus_core,rimms/jubatus,elkingtonmcb/jubatus,jubatus/jubatus,mathn/jubatus,rimms/jubatus_core,Asuka52/jubatus,rimms/jubatus_core,gintenlabo/jubatus_core,rimms/jubatus_core,kmaehashi/jubatus_core,kumagi/jubatus_core,rimms/jubatus_core,kumagi/jubatus_core,rimms/jubatus,gintenlabo/jubatus_core,roselleebarle04/jubatus,gintenlabo/jubatus_core,gintenlabo/jubatus_core,gintenlabo/jubatus