indexed BLOBs for ROW_FORMAT=DYNAMIC and ROW_FORMAT_COMPRESSED tables
(Bug #52746). In these tables, the locally stored prefix of a BLOB can
be as small as 20 bytes (BTR_EXTERN_FIELD_REF_SIZE). ROW_FORMAT=REDUNDANT
and ROW_FORMAT=COMPACT store a prefix of 768 bytes (REC_MAX_INDEX_COL_LEN).

trx_undo_rec_get_col_val(): Relax the ut_ad() assertion and add a
reference to dtuple_convert_big_rec().

trx_undo_rec_get_partial_row(): Relax the ut_a() assertion that
prompted Bug #52746.