Fix descriptor panic (#53)

* Provide fuzzer for descriptor

* Don't panic when the length of a descriptor is 0

descriptor.go

@@ -2002,6 +2002,10 @@ func calcDescriptorLength(d *Descriptor) uint8 {
 		return calcDescriptorUserDefinedLength(d.UserDefined)
 	}
 
+	if d.Length == 0 {
+		return 0
+	}
+
 	switch d.Tag {
 	case DescriptorTagAC3:
 		return calcDescriptorAC3Length(d.AC3)
@@ -2068,6 +2072,10 @@ func writeDescriptor(w *astikit.BitsWriter, d *Descriptor) (int, error) {
 
 	written := int(length) + 2
 
+	if d.Length == 0 {
+		return written, nil
+	}
+
 	if d.Tag >= 0x80 && d.Tag <= 0xfe {
 		return written, writeDescriptorUserDefined(w, d.UserDefined)
 	}

descriptor_test.go

@@ -2,9 +2,10 @@ package astits
 
 import (
 	"bytes"
+	"testing"
+
 	"github.com/asticode/go-astikit"
 	"github.com/stretchr/testify/assert"
-	"testing"
 )
 
 var descriptors = []*Descriptor{{
@@ -703,3 +704,31 @@ func BenchmarkParseDescriptor(b *testing.B) {
 		})
 	}
 }
+
+func FuzzDescriptor(f *testing.F) {
+	bufExpected := bytes.Buffer{}
+	bufExpected.Write([]byte{0x00, 0x00}) // reserve two bytes for length
+	wExpected := astikit.NewBitsWriter(astikit.BitsWriterOptions{Writer: &bufExpected})
+
+	for _, tc := range descriptorTestTable {
+		tc.bytesFunc(wExpected)
+	}
+
+	descLen := uint16(bufExpected.Len() - 2)
+	descBytes := bufExpected.Bytes()
+	descBytes[0] = byte(descLen>>8) | 0b11110000 // program_info_length is preceded by 4 reserved bits
+	descBytes[1] = byte(descLen & 0xff)
+
+	f.Add(descBytes)
+
+	f.Fuzz(func(t *testing.T, b []byte) {
+		ds, err := parseDescriptors(astikit.NewBytesIterator(b))
+
+		if err == nil {
+			bufActual := bytes.Buffer{}
+			wActual := astikit.NewBitsWriter(astikit.BitsWriterOptions{Writer: &bufActual})
+
+			writeDescriptorsWithLength(wActual, ds)
+		}
+	})
+}

testdata/fuzz/FuzzDescriptor/6ae8bb726335d15a

@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("0\x060\x02000\x00")

testdata/fuzz/FuzzDescriptor/c18ae01ae2349b75

@@ -0,0 +1,2 @@
+go test fuzz v1
+[]byte("0\x060\x0200X\x00")