Fix LP BUG#714999

Analysis:
The crash in EXPLAIN resulted from an attempt to print the
name of the internal temporary table created to compute
distinct for the innermost subquery of the test case.
Such tables do not have a corresponding TABLE_LIST (table
reference), hence the crash. The reason for this was that
the subquery was executed as part of constant condition
evaluation before EXPLAIN attempts to print the table name.
During the subquery execution, the subquery JOIN_TAB and
its table are substituted by a temporary table in
make_simple_join.

Solution:
Similar to the analogous case for other Items than the
IS NULL function, do not evaluate expensive constant
conditions.

sql/sql_select.cc

@@ -10966,7 +10966,7 @@ remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value)
 	}
       }
     }
-    if (cond->const_item())
+    if (cond->const_item() && !cond->is_expensive())
     {
       *cond_value= eval_const_cond(cond) ? Item::COND_TRUE : Item::COND_FALSE;
       return (COND*) 0;