Fix a bug that closed grn_ctx may be reused in UDF
Fix a bug that closed grn_ctx may be reused in UDF

[groonga-dev,03640]

Reported by Hironori Matsubara. Thanks!!!

yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,yoku0825/mroonga