MDEV-22728: SIGFPE in Unique::get_cost_calc_buff_size from prepare_search_best_index_intersect on optimized builds

For low sort_buffer_size, in the cost calculation of using the Unique object the elements in the tree were evaluated to 0, make sure to have atleast 1 element in the Unique tree.

Also for the function Unique::get allocate memory for atleast MERGEBUFF2+1 keys.