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