protcol/rtp: panic in Payload if length is not sufficiently long to be RTP packet.

protocol/rtp/parse.go

@@ -37,6 +37,9 @@ const badVer = "incompatible RTP version"
 // Payload returns the payload from an RTP packet provided the version is
 // compatible, otherwise an error is returned.
 func Payload(d []byte) ([]byte, error) {
+	if len(d) < defaultHeadSize {
+		panic("invalid RTP packet length")
+	}
 	if version(d) != rtpVer {
 		return nil, errors.New(badVer)
 	}