mysql57: add NULL check in reset()
mysql57: add NULL check in reset()

Because table->pos_in_table_list may be NULL.

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