Fix for bug#20648 We introduce a new field method for knowing "real size", and we now in archive null unused bits of a row to null before writing. 


sql/field.cc:
  data_length for field needs to be calculated directly.
sql/field.h:
  The new method data_length() returns the "real" length of the field.
sql/ha_archive.cc:
  Before a write_row() archive nulls space beyond the size of the row in varchars to make sure that compression only sees NULL.