mysql51: fix crash bug
mysql51: fix crash bug

MySQL 5.1 doesn't have utf8mb4. We need to check its availability.

[groonga-dev,01069]

Reported by wakisuke. Thanks!!!

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