When there is conjunction of conds, the substitute_for_best_equal_field()
will call the eliminate_item_equal() function in loop to build final
expression. But if eliminate_item_equal() finds that some cond will always
evaluate to 0, then that cond will be substituted by Item_int with value ==
0. In this case on the next iteration eliminate_item_equal() will get that 
Item_int and treat it as Item_cond. This is leads to memory corruption and
server crash on cleanup phase.

To the eliminate_item_equal() function was added DBUG_ASSERT for checking
that all items treaten as Item_cond are really Item_cond.
The substitute_for_best_equal_field() now checks that if
eliminate_item_equal() returns Item_int and it's value is 0 then this 
value is returned as the result of whole conjunction.