len was containing garbage, since vctempl->mysql_col_offset was
containing old value while calling row_mysql_store_col_in_innobase_format
from innobase_get_computed_value().

It was not updated after the first ALTER TABLE call, because it's INPLACE
logic considered there's nothing to update, and exited immediately from
ha_innobase::inplace_alter_table().

However, vcol metadata needs an update, since vcols structure is changed
in mysql record.

The regression was introduced by 12614af1. There, refcount==1 condition
was removed, which turned out to be crucial, though racy. The idea was to
update vc_templ after each (sequencing) ALTER TABLE.

We should do the same another way, and there may be a plenty of solutions,
but the simplest one is to add a following condition:
  if vcol structure is changed, drop vc_templ; it will be recreated on next
  ha_innobase::open() call.

in prepare_inplace_alter_table. It is safe, since innodb inplace changes
require at least HA_ALTER_INPLACE_SHARED_LOCK_AFTER_PREPARE, which
guarantee MDL_EXCLUSIVE on this stage.

alter_templ_needs_rebuild() also has to track the columns not indexed, to
keep vc_templ correct.

Note that vc_templ is always kept constructed and available after
ha_innobase::open() call, even on INSERT, though no virtual columns are
evaluated during that statement
inside innodb.

In the test case suplied, it will be recreated on the second ALTER TABLE.