do not try to delete non-existent object in cache (#7560)

handle cache cleanup correctly when backend object was deleted.

Fixes: #7558

cmd/disk-cache.go

@@ -200,10 +200,13 @@ func (c cacheObjects) GetObjectNInfo(ctx context.Context, bucket, object string,
 		return cacheReader, nil
 	} else if err != nil {
 		if _, ok := err.(ObjectNotFound); ok {
+			if cacheErr == nil {
 				cacheReader.Close()
-			// Delete cached entry if backend object was deleted.
+				// Delete cached entry if backend object
+				// was deleted.
 				dcache.Delete(ctx, bucket, object)
 			}
+		}
 		return nil, err
 	}