ByteToMessageDecoder.handlerRemoved(...) should only call fireChannelReadComplete() if fireChannelRead(...) was called before (#9211)

Motivation:

At the moment ByteToMessageDecoder always calls fireChannelReadComplete() when the handler is removed from the pipeline and the cumulation buffer is not null. We should only call it when we also call fireChannelRead(...), which only happens if the cumulation buffer is not null and readable.

Modifications:

Only call fireChannelReadComplete() if fireChannelRead(...) is called before during removal of the handler.

Result:

More correct semantics

codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java

@@ -235,18 +235,16 @@ public abstract class ByteToMessageDecoder extends ChannelHandlerAdapter impleme
         if (buf != null) {
             // Directly set this to null so we are sure we not access it in any other method here anymore.
             cumulation = null;
-
+            numReads = 0;
             int readable = buf.readableBytes();
             if (readable > 0) {
                 ByteBuf bytes = buf.readBytes(readable);
                 buf.release();
                 ctx.fireChannelRead(bytes);
+                ctx.fireChannelReadComplete();
             } else {
                 buf.release();
             }
-
-            numReads = 0;
-            ctx.fireChannelReadComplete();
         }
         handlerRemoved0(ctx);
     }