bugfix: don't calculate vcols if doing keyread
bugfix: don't calculate vcols if doing keyread

old code didn't calculate vcols that were part of keyread,
but calculated other vcols. It was wrong - there was no guarantee
that vcol's base columns were part of keyread.

Technically it's possible for the vcol not be a part of keyread,
but all its base columns being part of keyread. But currently the
optimizer doesn't do that, keyread is only used if it covers
all columns used in the query.

This fixes crashes of vcol.vcol_trigger_sp_innodb

natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2