decryptRTCP() should check min length

[yuanchao0310]

Your environment.

• Version: master
• Browser: include version
• Other Information - stacktraces, related issues, suggestions how to fix, links for us to have context

What did you do?

What did you expect?

What happened?

func (c *Context) decryptRTCP(dst, encrypted []byte) ([]byte, error) {
	out := allocateIfMismatch(dst, encrypted)

	authTagLen, err := c.cipher.rtcpAuthTagLen()
	if err != nil {
		return nil, err
	}
	aeadAuthTagLen, err := c.cipher.aeadAuthTagLen()
	if err != nil {
		return nil, err
	}
	tailOffset := len(encrypted) - (authTagLen + srtcpIndexSize)

	if tailOffset < aeadAuthTagLen {
		return nil, fmt.Errorf("%w: %d", errTooShortRTCP, len(encrypted))
	} else if isEncrypted := encrypted[tailOffset] >> 7; isEncrypted == 0 {
		return out, nil
	}

	index := c.cipher.getRTCPIndex(encrypted)
	ssrc := binary.BigEndian.Uint32(encrypted[4:])

	s := c.getSRTCPSSRCState(ssrc)
	markAsValid, ok := s.replayDetector.Check(uint64(index))
	if !ok {
		return nil, &duplicatedError{Proto: "srtcp", SSRC: ssrc, Index: index}
	}

	out, err = c.cipher.decryptRTCP(out, encrypted, index, ssrc)
	if err != nil {
		return nil, err
	}

	markAsValid()
	return out, nil
}

srtcp data length min length should be 8 + aeadAuthTagLen + authTagLen + srtcpIndexSize
8 is rtcp header(4 bytes + 4 bytes ssrc)

assume a srtcp packet is 4 + aeadAuthTagLen + authTagLen + srtcpIndexSize, then

ssrc := binary.BigEndian.Uint32(encrypted[4:])

will get a wrong value. so need check if len(encrypted) >= min length in decryptRTCP function