mariabackup deallocated uninitialized
write_filt_ctxt.u.wf_incremental_ctxt in xtrabackup_copy_datafile() when
some table should be skipped due to parsed DDL redo log record.
backup_release(): New function, refactored from backup_finish().
Release some resources that may have been acquired by backup_startup()
and should be released even after a failed operation.
xtrabackup_backup_low(): Refactored from xtrabackup_backup_func().
xtrabackup_backup_func(): Always call backup_release() after calling
backup_start().
