mariadb 10: fix a crash bug
mariadb 10: fix a crash bug

https://gitter.im/groonga/ja?at=57746049265214c130a655ed

Reported by yoyoshifgs. Thanks!!!

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