Assertion `table->field[0]->ptr >= table->record[0] &&
table->field[0]->ptr <= table->record[0] + table->s->reclength' failed in
handler::assert_icp_limitations.

table->move_fields has some limitations:
1. It cannot be used in cascade
2. It should always have a restoring pair.

Rule 1 is covered by assertions in handler::assert_icp_limitations
and handler::ptr_in_record (commit 30894fe9).

Rule 2 should be manually maintained with care. Hopefully, the rule 1 assertions
may sometimes help as well.

In ha_myisam::repair, both rules are broken. table->move_fields is used
asymmetrically there: it is set on every param->fix_record call
(i.e. in compute_vcols) but is restored only once, in the end of repair.

The reason to updating field ptr's for every call is that compute_vcols can
(supposedly) be called in parallel, that is, with the same table, but different
records.

The condition to "unmove" the pointers in ha_myisam::restore_vcos_after_repair
is incorrect, when stored vcols are available, and myisam stores a VIRTUAL field
if it's the only field in the table (the record cannot be of zero length).

This patch solves the problem by "unmoving" the pointers symmetrically, in
compute_vcols. That is, both rules will be preserved maintained.