fix(downloader): order DiskCache.Get checks for overlayfs empty dirs

Signed-off-by: Sumit Solanki <sumit.solanki@ibm.com>

pkg/downloader/cache.go

@@ -59,15 +59,17 @@ func (c *DiskCache) Get(key [sha256.Size]byte, cacheType string) (string, error)
 	if err != nil {
 		return "", err
 	}
-	// Empty files treated as not exist because there is no content.
-	if fi.Size() == 0 {
-		return p, os.ErrNotExist
-	}
 	// directories should never happen unless something outside helm is operating
 	// on this content.
 	if fi.IsDir() {
 		return p, errors.New("is a directory")
 	}
+	// Empty files treated as not exist because there is no content.
+	// IsDir must be checked first: some filesystems (e.g. overlay) report
+	// directory size as 0.
+	if fi.Size() == 0 {
+		return p, os.ErrNotExist
+	}
 	return p, nil
 }