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: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Message-id: 1416566940-4430-3-git-send-email-stefanha@redhat.com
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

blockdev.c

@@ -1489,8 +1489,6 @@ static void drive_backup_prepare(BlkTransactionState *common, Error **errp)
                      &local_err);
     if (local_err) {
         error_propagate(errp, local_err);
-        state->bs = NULL;
-        state->job = NULL;
         return;
     }