row_build_index_entry_low(): Assert that when the value of a
virtual column is not available, this can only happen when
the index creation was completed but not committed yet.

This change is not fixing any bug, making a debug assertion
stricter, so that bugs can be caught in the future.

Ultimately, we should change the InnoDB undo log format so that
all actual secondary index keys are stored there, also for
virtual or spatial indexes. In that way, purge and rollback would
be more straightforward.