- Make convert_zerofill_number_to_string() take into account that the 
  constant it is converting may evaluate to NULL.


mysql-test/r/subselect.result:
  BUG#36139 "float, zerofill, crash with subquery"
  - Testcase
mysql-test/t/subselect.test:
  BUG#36139 "float, zerofill, crash with subquery"
  - Testcase