trx-undo_prev_version_build(): Pass offsets==NULL to
rec_set_field_extern_bits().

rec_set_field_extern_bits(), rec_set_field_extern_bits_new():
Accept offsets==NULL.

row_upd_rec_in_place(): Remove the bogus comment that the function
would only be invoked on a clustered index.  Remove the related
debug assertion.