row_build_index_entry_low(): ext does not contain virtual columns.

row_upd_store_v_row(): Copy virtual column values

This is based on the following fix in MySQL 5.7.24:

commit 4ec2158bec73f1582501c4b3e3de250fed9edc9a
Author: Sachin Agarwal <sachin.z.agarwal@oracle.com>
Date:   Fri Aug 24 14:44:13 2018 +0530

    Bug #27968952 INNODB CRASH/CORRUPTION WITH TEXT PREFIX INDEXES

    Problem:
    There are two problems:
      1. If there is one secondary index on extenally
    stored column and another seconday index on virtual column (whose
    base column is not externally stored). then while updating seconday
    index on vitrual column, virtual column data is replaced by
    externally stoared column.
      2. In row update operation, node->row contains
    shallow copy of virtual data fields. While building an update vector
    containing all the fields to be modified, compute virtual column.
    which may causes change in virtual data fields in node->row.

    In both the above cases, while updating seconday index on virtual
    column, couldn't find the row and hit an explicite assert inside
    ROW_NOT_FOUND.

    Fix:
    1. Added check if column is virtual then its ext flag should be ZERO
    and virtual column data will not be replaced by offset column data.
    2. Deep copy of virtual data fields for node->row.

    RB: #20382
    Reviewed by : Jimmy.Yang@oracle.com