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

@@ -200,9 +200,12 @@ func (c cacheObjects) GetObjectNInfo(ctx context.Context, bucket, object string,
 		return cacheReader, nil
 	} else if err != nil {
 		if _, ok := err.(ObjectNotFound); ok {
-			cacheReader.Close()
-			// Delete cached entry if backend object was deleted.
-			dcache.Delete(ctx, bucket, object)
+			if cacheErr == nil {
+				cacheReader.Close()
+				// Delete cached entry if backend object
+				// was deleted.
+				dcache.Delete(ctx, bucket, object)
+			}
 		}
 		return nil, err
 	}