fixed issue with SourceDebugExtension attribute

src/org/objectweb/asm/ClassWriter.java

@@ -556,8 +556,7 @@ public class ClassWriter implements ClassVisitor {
             sourceFile = newUTF8(file);
         }
         if (debug != null) {
-            sourceDebug = new ByteVector();
-            sourceDebug.putUTF8(debug);
+            sourceDebug = new ByteVector().putUTF8(debug);
         }
     }
 
@@ -676,7 +675,7 @@ public class ClassWriter implements ClassVisitor {
         }
         if (sourceDebug != null) {
             ++attributeCount;
-            size += sourceDebug.length;
+            size += sourceDebug.length + 4;
             newUTF8("SourceDebugExtension");
         }
         if (enclosingMethodOwner != 0) {
@@ -758,9 +757,9 @@ public class ClassWriter implements ClassVisitor {
             out.putShort(newUTF8("SourceFile")).putInt(2).putShort(sourceFile);
         }
         if (sourceDebug != null) {
-            int len = sourceDebug.length;
+            int len = sourceDebug.length - 2;
             out.putShort(newUTF8("SourceDebugExtension")).putInt(len);
-            out.putByteArray(sourceDebug.data, 0, len);
+            out.putByteArray(sourceDebug.data, 2, len);
         }
         if (enclosingMethodOwner != 0) {
             out.putShort(newUTF8("EnclosingMethod")).putInt(4);