trx_undo_rec_copy(): Return nullptr if the undo record is corrupted.

trx_undo_rec_get_undo_no(): Define inline with the declaration.

trx_purge_dummy_rec: Replaced with a -1 pointer.

row_undo_rec_get(), UndorecApplier::apply_undo_rec(): Check
if trx_undo_rec_copy() returned nullptr.

trx_purge_get_next_rec(): Return nullptr upon encountering any
corruption, to signal the end of purge.