* get_rec_bits() was always reading two bytes, even if the
  bit field contained only of one byte
* In various places the code used field->pack_length() bytes
  starting from field->ptr, while it should be field->pack_length_in_rec()
* Field_bit::key_cmp and Field_bit::cmp_max passed field_length as
  an argument to memcmp(), but field_length is the number of bits!