[netty#3368] Ensure ByteBuf is not release two times

Motivation:

As the ByteBuf is not set to null after release it we may try to release it again in handleReadException()

Modifications:

-  set ByteBuf to null to avoid another byteBuf.release() to be called in handleReadException()

Result:

No IllegalReferenceCountException anymore

transport/src/main/java/io/netty/channel/nio/AbstractNioByteChannel.java

@@ -116,6 +116,7 @@ public void read() {
                     if (localReadAmount <= 0) {
                         // not was read release the buffer
                         byteBuf.release();
+                        byteBuf = null;
                         close = localReadAmount < 0;
                         break;
                     }