is_last_prefix <= 0, file .\opt_range.cc.

SELECT ... GROUP BY bit field failed with an assertion if the
bit length of that field was not divisible by 8.



sql/key.cc:
  Fixed bug #32556.
  Copying of "uneven" bits of a bit field was duplicated in the
  key_copy() and in the Field_bit::get_key_image().
  So, instead of copying of the rest of a bit field, 
  Field_bit::get_key_image() copied "uneven" bits to key image again,
  and the lowest field byte was not copied to key at all.
  
  Duplicated code has been removed from the key_copy function.
mysql-test/t/type_bit.test:
  Added test case for bug #32556.
mysql-test/r/type_bit.result:
  Added test case for bug #32556.