fix: parsing certificateReference

Parsing certificateReference to integer may cause the value range to be
exceeded. Therefore, it is now parsed to BigInteger

pom.xml

@@ -3,7 +3,7 @@
 
 	<groupId>de.tsenger</groupId>
 	<artifactId>vdstools</artifactId>
-	<version>0.3.1</version>
+	<version>0.3.2</version>
 	<name>VDS Tools java library</name>
 	<description>A library to parse, verify and build/encode Visible Digital Seals (VDS)</description>
 	<packaging>jar</packaging>

src/main/java/de/tsenger/vdstools/seals/DigitalSeal.java

@@ -1,6 +1,7 @@
 package de.tsenger.vdstools.seals;
 
 import java.io.IOException;
+import java.math.BigInteger;
 import java.security.cert.X509Certificate;
 import java.time.LocalDate;
 import java.util.EnumMap;
@@ -77,8 +78,8 @@ public String getIssuingCountry() {
      * @return Formated SignerCertRef all UPPERCASE
      */
     public String getSignerCertRef() {
-        int certRefInteger = Integer.decode("0x" + vdsHeader.certificateReference);
-        return String.format("%s%x", vdsHeader.signerIdentifier, certRefInteger).toUpperCase();
+    	BigInteger certRefInteger = new BigInteger(vdsHeader.certificateReference, 16);
+    	return String.format("%s%x", vdsHeader.signerIdentifier, certRefInteger).toUpperCase();
     }
 
     public String getSignerIdentifier() {