MDEV-6484: Assertion `tab->ref.use_count' failed on query with joins, constant table, multi-part key

- test_if_skip_sort_order()/create_ref_for_key() may change table 
  access from EQ_REF(index1) to REF(index2). 
- Doing so doesn't make much sense from optimization POV, but since 
  they are doing it, they should update tab->read_record.unlock_row
  accordingly.

sql/sql_select.cc

@@ -8178,6 +8178,9 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
   }
   else
     j->type=JT_EQ_REF;
+
+  j->read_record.unlock_row= (j->type == JT_EQ_REF)? 
+                             join_read_key_unlock_row : rr_unlock_row; 
   DBUG_RETURN(0);
 }