innobase/rem/rem0rec.c:
  rec_set_nth_field_extern_bit_new(): Fix a bug
  (read the "extern" flag from the correct position).
  
  rec_set_nth_field_extern_bit_new(),
  rec_convert_dtuple_to_rec_new(),
  rec_copy_prefix_to_buf(): Eliminate variables, reduce branching,
  optimize the handling of the null flags.