Fix SIGSEGV when calling Find on empty Queue

Resolves pion/webrtc#2740

pkg/jitterbuffer/priority_queue.go

@@ -51,14 +51,6 @@ func newNode(val *rtp.Packet, priority uint16) *node {
 // Find a packet in the queue with the provided sequence number,
 // regardless of position (the packet is retained in the queue)
 func (q *PriorityQueue) Find(sqNum uint16) (*rtp.Packet, error) {
-	if q.next.priority == sqNum {
-		return q.next.val, nil
-	}
-
-	if sqNum < q.next.priority {
-		return nil, ErrInvalidOperation
-	}
-
 	next := q.next
 	for next != nil {
 		if next.priority == sqNum {

pkg/jitterbuffer/priority_queue_test.go

@@ -12,6 +12,7 @@ import (
 
 func TestPriorityQueue(t *testing.T) {
 	assert := assert.New(t)
+
 	t.Run("Appends packets in order", func(*testing.T) {
 		pkt := &rtp.Packet{Header: rtp.Header{SequenceNumber: 5000, Timestamp: 500}, Payload: []byte{0x02}}
 		q := NewQueue()
@@ -22,6 +23,7 @@ func TestPriorityQueue(t *testing.T) {
 		assert.Equal(q.next.priority, uint16(5000))
 		assert.Equal(q.next.next.priority, uint16(5004))
 	})
+
 	t.Run("Appends many in order", func(*testing.T) {
 		q := NewQueue()
 		for i := 0; i < 100; i++ {
@@ -40,6 +42,7 @@ func TestPriorityQueue(t *testing.T) {
 		assert.Equal(q.next.priority, uint16(5012))
 		assert.Equal(last.priority, uint16(5012+99))
 	})
+
 	t.Run("Can remove an element", func(*testing.T) {
 		pkt := &rtp.Packet{Header: rtp.Header{SequenceNumber: 5000, Timestamp: 500}, Payload: []byte{0x02}}
 		q := NewQueue()
@@ -55,6 +58,7 @@ func TestPriorityQueue(t *testing.T) {
 		nextPop, _ := q.Pop()
 		assert.Equal(nextPop.SequenceNumber, uint16(5012))
 	})
+
 	t.Run("Appends in order", func(*testing.T) {
 		q := NewQueue()
 		for i := 0; i < 100; i++ {
@@ -67,6 +71,7 @@ func TestPriorityQueue(t *testing.T) {
 		assert.Equal(uint16(101), q.Length())
 		assert.Equal(q.next.priority, uint16(5000))
 	})
+
 	t.Run("Can find", func(*testing.T) {
 		q := NewQueue()
 		for i := 0; i < 100; i++ {
@@ -77,3 +82,22 @@ func TestPriorityQueue(t *testing.T) {
 		assert.Equal(err, nil)
 	})
 }
+
+func TestPriorityQueue_Find(t *testing.T) {
+	packets := NewQueue()
+
+	packets.Push(&rtp.Packet{
+		Header: rtp.Header{
+			SequenceNumber: 1000,
+			Timestamp:      5,
+			SSRC:           5,
+		},
+		Payload: []uint8{0xA},
+	}, 1000)
+
+	_, err := packets.PopAt(1000)
+	assert.NoError(t, err)
+
+	_, err = packets.Find(1001)
+	assert.Error(t, err)
+}