- In eliminate_item_equal(), we could end up in a situation where:
  = The multiple equality has a constant C1 (and so it is the "head item")
  = The join order was such that we've generated "sj_inner_table1=C1" equality,
    and now are looking to generate "sj_inner_table2_=..." equality. 
  When looking for what should be the other member of equality, we run

      Item *head_item= current_sjm? current_sjm_head: head;
  
  which sees current_sjm!=NULL, and takes current_sjm_head (which is NULL because 
  the constant C1 is the head for all cases).

- Fixed in a trivial way: take current_sjm_head if we don't have constant.