crypto: add length check to hacky megolm message index parser

crypto/encryptmegolm.go

@@ -91,11 +91,16 @@ func IsShareError(err error) bool {
 }
 
 func ParseMegolmMessageIndex(ciphertext []byte) (uint, error) {
+	if len(ciphertext) == 0 {
+		return 0, fmt.Errorf("empty ciphertext")
+	}
 	decoded := make([]byte, base64.RawStdEncoding.DecodedLen(len(ciphertext)))
 	var err error
 	_, err = base64.RawStdEncoding.Decode(decoded, ciphertext)
 	if err != nil {
 		return 0, err
+	} else if len(decoded) < 2+binary.MaxVarintLen64 {
+		return 0, fmt.Errorf("decoded ciphertext too short: %d bytes", len(decoded))
 	} else if decoded[0] != 3 || decoded[1] != 8 {
 		return 0, fmt.Errorf("unexpected initial bytes %d and %d", decoded[0], decoded[1])
 	}