- Release build: Fix compiler warning about unused variable

- Debug build: make DBUG_ASSERT statements really look at the first table

One can't expect cursors to work with semi-join subqueries anyway, 
though (which isn't a problem because cursors are not used anywhere)

sql/sql_cursor.cc

@@ -363,7 +363,6 @@ Sensitive_cursor::open(JOIN *join_arg)
   join= join_arg;
   THD *thd= join->thd;
   /* First non-constant table */
-  JOIN_TAB *join_tab= join->join_tab + join->const_tables;
   DBUG_ENTER("Sensitive_cursor::open");
 
   join->change_result(result);
@@ -394,13 +393,16 @@ Sensitive_cursor::open(JOIN *join_arg)
       tab->next_select= sub_select;
   }
 
-  DBUG_ASSERT(join_tab->table->reginfo.not_exists_optimize == 0);
-  DBUG_ASSERT(join_tab->not_used_in_distinct == 0);
+#ifndef DBUG_OFF
+  JOIN_TAB *first_tab= first_linear_tab(join, WITHOUT_CONST_TABLES);
+  DBUG_ASSERT(first_tab->table->reginfo.not_exists_optimize == 0);
+  DBUG_ASSERT(first_tab->not_used_in_distinct == 0);
   /*
     null_row is set only if row not found and it's outer join: should never
     happen for the first table in join_tab list
   */
-  DBUG_ASSERT(join_tab->table->null_row == 0);
+  DBUG_ASSERT(first_tab->table->null_row == 0);
+#endif
   DBUG_RETURN(0);
 }