column is used for ORDER BY

Problem: filesort isn't meant for null length sort data
(e.g. char(0)), that leads to a server crash.

Fix: disregard sort order if sort data record length is 0 (nothing
to sort).


mysql-test/r/select.result:
  Fix for bug#49897: crash in ptr_compare when char(0) NOT NULL 
  column is used for ORDER BY
    - test result.
mysql-test/t/select.test:
  Fix for bug#49897: crash in ptr_compare when char(0) NOT NULL 
  column is used for ORDER BY
    - test case.
sql/filesort.cc:
  Fix for bug#49897: crash in ptr_compare when char(0) NOT NULL 
  column is used for ORDER BY
    - assert added as filesort cannot handle null length sort data.
sql/sql_select.cc:
  Fix for bug#49897: crash in ptr_compare when char(0) NOT NULL 
  column is used for ORDER BY
    - don't sort null length data e.g. in case of ORDER BY CHAR(0).