HttpPostRequestEncoder may return a slice which is not retained.

Motivation:

We miss to retain a slice before return it to the user and so an reference count error may accour later on.

Modifications:

Use readRetainedSlice(...) and so ensure we retain the buffer before hand it of to the user.

Result:

Fixes [#6626].

codec-http/src/main/java/io/netty/handler/codec/http/multipart/HttpPostRequestEncoder.java

@@ -894,9 +894,7 @@ public class HttpPostRequestEncoder implements ChunkedInput<HttpContent> {
     private ByteBuf fillByteBuf() {
         int length = currentBuffer.readableBytes();
         if (length > HttpPostBodyUtil.chunkSize) {
-            ByteBuf slice = currentBuffer.slice(currentBuffer.readerIndex(), HttpPostBodyUtil.chunkSize);
+            return currentBuffer.readRetainedSlice(HttpPostBodyUtil.chunkSize);
-            currentBuffer.skipBytes(HttpPostBodyUtil.chunkSize);
-            return slice;
         } else {
             // to continue
             ByteBuf slice = currentBuffer;