use ByteBuffer.duplicate() in setDataPointer()

This avoids modifying the position of value.buffer.

Author: dwrensha

runtime/src/main/java/org/capnproto/WireHelpers.java

@@ -850,14 +850,13 @@ static Data.Builder setDataPointer(int refOffset,
                                        SegmentBuilder segment,
                                        Data.Reader value) {
         Data.Builder builder = initDataPointer(refOffset, segment, value.size);
-        int originalSrcPosition = value.buffer.position();
-        int originalDstPosition = builder.buffer.position();
+
+        ByteBuffer slice = value.buffer.duplicate();
+        slice.position(value.offset);
+        slice.limit(value.offset + value.size);
+
         builder.buffer.position(builder.offset);
-        //# copy from src offset until src end
-        builder.buffer.put(value.buffer);
-        //# restore original positions
-        builder.buffer.position(originalDstPosition);
-        value.buffer.position(originalSrcPosition);
+        builder.buffer.put(slice);
         return builder;
     }