update_virtual_field() is called as part of index rebuild in
ha_myisam::repair() (MDEV-5800) which is done on bulk INSERT finish.

Assertion in update_virtual_field() was put as part of MDEV-16222
because update_virtual_field() returns in_use->is_error(). The idea:
wrongly mixed semantics of error status before update_virtual_field()
and the status returned by update_virtual_field(). The former can
falsely influence the latter.