Fix keyframe detection for VP8 and VP9.

codecs/codecs.go

@@ -266,10 +266,10 @@ func PacketFlags(codec string, buf []byte) (Flags, error) {
 		if err != nil {
 			return flags, err
 		}
-
 		flags.Start = vp8.S != 0 && vp8.PID == 0
 		flags.End = packet.Marker
-		flags.Keyframe = vp8.S != 0 && (vp8.Payload[0]&0x1) == 0
+		flags.Keyframe = vp8.S != 0 && vp8.PID == 0 &&
+			len(vp8.Payload) > 0 && (vp8.Payload[0]&0x1) == 0
 		flags.Pid = vp8.PictureID
 		flags.Tid = vp8.TID
 		flags.TidUpSync = flags.Keyframe || vp8.Y == 1
@@ -289,7 +289,8 @@ func PacketFlags(codec string, buf []byte) (Flags, error) {
 		}
 		flags.Start = vp9.B
 		flags.End = vp9.E
-		if (vp9.Payload[0] & 0xc0) == 0x80 {
+		if vp9.B && len(vp9.Payload) > 0 &&
+			(vp9.Payload[0] & 0xc0) == 0x80 {
 			profile := (vp9.Payload[0] >> 4) & 0x3
 			if profile != 3 {
 				flags.Keyframe = (vp9.Payload[0] & 0xC) == 0