Server crashes in Field::register_field_in_read_map upon select from
partitioned table with indexed by prefix virtual column.

After several read-mark fixes a problem has surfaced:
Since KEY (c(10),a) uses only a prefix of c, a new field is created,
duplicated from table->field[3], with a new length. However,
vcol_inco->expr is not copied.

Therefore, (*key_info)->key_part[i].field->vcol_info->expr was left NULL
in ha_partition::index_init().

Solution: initialize vcols before key initialization

Also key initialization is moved to a function.