Added extra check for NULL destination_file pointer, to help catch race with unlink().
Added extra check for NULL destination_file pointer, to help catch race with unlink().

kbauskar/Percona-TokuBackup,kbauskar/Percona-TokuBackup,guiquanz/Percona-TokuBackup,kbauskar/Percona-TokuBackup,kbauskar/Percona-TokuBackup,dbpercona/Percona-TokuBackup,dbpercona/Percona-TokuBackup,dbpercona/Percona-TokuBackup,tplavcic/Percona-TokuBackup,percona/Percona-TokuBackup,tplavcic/Percona-TokuBackup,percona/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,guiquanz/Percona-TokuBackup,percona/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,tplavcic/Percona-TokuBackup,guiquanz/Percona-TokuBackup,percona/Percona-TokuBackup,tplavcic/Percona-TokuBackup,dbpercona/Percona-TokuBackup,Jun-Yuan/Percona-TokuBackup,guiquanz/Percona-TokuBackup