This bug was originally repeated on 10.4 after defining a UNIQUE KEY
on a TEXT column, which is implemented by MDEV-371 by creating the
index on a hidden virtual column.
While row_vers_vc_matches_cluster() is executing in a purge thread
to find out if an index entry may be removed in a secondary index
that comprises a virtual column, another purge thread may process
the undo log record that this check is interested in, and write
a null BLOB pointer in that record. This would trip the assertion.
To prevent this from occurring, we must propagate the 'missing BLOB'
error up the call stack.
row_upd_ext_fetch(): Return NULL when the error occurs.
row_upd_index_replace_new_col_val(): Return whether the previous
version was built successfully.
row_upd_index_replace_new_col_vals_index_pos(): Check the error
result. Yes, we would intentionally crash on this error if it
occurs outside the purge thread.
row_upd_index_replace_new_col_vals(): Check for the error condition,
and simplify the logic.
trx_undo_prev_version_build(): Check for the error condition.
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