Extract Hex class resource logic to a separate method (#20)

* feat: Extract Hex class resource logic to a separate method

Sorry for the weird commit message, I couldn't find a way to word it better. Anyways..

This commit extracts the `getResourceAsStream` call to a separate, protected method so it can be overrided, and we can provide the Hex class from a different source.

* fix: Check if Hex class stream is null

dex-translator/src/main/java/com/googlecode/d2j/dex/Dex2Asm.java

@@ -561,8 +561,17 @@ public void convertClass(int dexVersion, DexClassNode classNode, ClassVisitorFac
     private static final Set<String> HEX_DECODE_METHODS =
             new HashSet<>(Arrays.asList("decode_J", "decode_I", "decode_S", "decode_B"));
 
+    protected InputStream getHexClassAsStream() {
+        return Dex2Asm.class.getResourceAsStream("/" + HEX_CLASS_LOCATION + ".class");
+    }
+
     private void addHexDecodeMethod(ClassVisitor outCV, String className, String hexDecodeMethodNameBase) {
-        try (InputStream is = Dex2Asm.class.getResourceAsStream("/" + HEX_CLASS_LOCATION + ".class")) {
+        InputStream hexClassStream = getHexClassAsStream();
+        if (hexClassStream == null) {
+            return;
+        }
+
+        try (InputStream is = hexClassStream) {
             ClassReader cr = new ClassReader(is);
             cr.accept(new ClassVisitor(Constants.ASM_VERSION) {
                 @Override