blockdev: drop unnecessary DriveBackupState field assignment

blockdev: drop unnecessary DriveBackupState field assignment

drive_backup_prepare() assigns DriveBackupState fields to NULL in the
error path.  This is unnecessary because the DriveBackupState is
allocated using g_malloc0() and other functions like
external_snapshot_prepare() already rely on this.

Do not explicitly assign fields to NULL so that the error path is
concise and does not require modification when fields are added to
DriveBackupState.

Signed-off-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 1416566940[email protected]
Signed-off-by: Kevin Wolf <[email protected]>