an error, asserts server

In case of a fatal error during filesort in find_all_keys() the error
was returned without the necessary handler uninitialization.
Fixed by changing the code so that handler uninitialization is performed
before returning the error.


mysql-test/r/delete.result:
  Added a test case for bug #31742.
mysql-test/t/delete.test:
  Added a test case for bug #31742.
sql/filesort.cc:
  In case of a fatal error in find_all_keys() do not return before doing
  the necessary handler uninitialization steps.