audio/vorbis: bug fix: error when a non-seeker source is given to Decode*

Closes #2252

audio/vorbis/vorbis.go

@@ -130,11 +130,13 @@ func decode(in io.Reader) (*decoded, int, int, error) {
 		posInBytes: 0,
 		decoder:    r,
 	}
-	if _, err := d.Read(make([]byte, 65536)); err != nil && err != io.EOF {
-		return nil, 0, 0, err
-	}
-	if _, err := d.Seek(0, io.SeekStart); err != nil {
-		return nil, 0, 0, err
+	if _, ok := in.(io.Seeker); ok {
+		if _, err := d.Read(make([]byte, 65536)); err != nil && err != io.EOF {
+			return nil, 0, 0, err
+		}
+		if _, err := d.Seek(0, io.SeekStart); err != nil {
+			return nil, 0, 0, err
+		}
 	}
 	return d, r.Channels(), r.SampleRate(), nil
 }

audio/vorbis/vorbis_test.go

@@ -16,6 +16,7 @@ package vorbis_test
 
 import (
 	"bytes"
+	"io"
 	"testing"
 
 	"github.com/jfreymuth/oggvorbis"
@@ -66,3 +67,21 @@ func TestTooShort(t *testing.T) {
 		t.Errorf("s.Length(): got: %d, want: %d", got, want)
 	}
 }
+
+type reader struct {
+	r io.Reader
+}
+
+func (r *reader) Read(buf []byte) (int, error) {
+	return r.r.Read(buf)
+}
+
+func TestNonSeeker(t *testing.T) {
+	bs := test_tooshort_ogg
+
+	s, err := vorbis.DecodeWithSampleRate(audioContext.SampleRate(), &reader{r: bytes.NewReader(bs)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	_ = s
+}