In commit 0e5a4ac2 (MDEV-15562)
we introduced was a bogus debug check failure that does not affect
the correctness of the release build.

With a fixed-length PRIMARY KEY, we do not have to recompute
the rec_get_offsets() after restarting the mini-transaction,
because the offsets of DB_TRX_ID,DB_ROLL_PTR are not going
to change.

row_undo_mod_clust(): Invoke rec_offs_make_valid() to keep the
debug check in page_zip_write_trx_id_and_roll_ptr() happy.

The scenario to reproduce this bug should be rather unlikely:
In the time frame when row_undo_mod_clust() has committed its
first mini-transaction and has not yet started the next one,
another mini-transaction must do something that causes the page
to be reorganized, split or merged.