FIX : Unpacking causes socks5proxy init failure (netty#9582)

Motivation:

Socks5InitialRequestDecoder does not correctly handle fragmentation

Modifications:

- Delete detection of not enough bytes as ReplyingDecoder already handles all of this correctly.
- Add unit test

Result:

Fixes netty#9574.

codec-socks/src/main/java/io/netty/handler/codec/socksx/v5/Socks5InitialRequestDecoder.java

@@ -56,9 +56,6 @@ protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) t
                 }
 
                 final int authMethodCnt = in.readUnsignedByte();
-                if (actualReadableBytes() < authMethodCnt) {
-                    break;
-                }
 
                 final Socks5AuthMethod[] authMethods = new Socks5AuthMethod[authMethodCnt];
                 for (int i = 0; i < authMethodCnt; i++) {

codec-socks/src/test/java/io/netty/handler/codec/socksx/v5/Socks5InitialRequestDecoderTest.java

@@ -0,0 +1,38 @@
+/*
+ * Copyright 2019 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+package io.netty.handler.codec.socksx.v5;
+
+import io.netty.buffer.Unpooled;
+import io.netty.channel.embedded.EmbeddedChannel;
+import io.netty.handler.codec.DecoderResult;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class Socks5InitialRequestDecoderTest {
+    @Test
+    public void testUnpackingCausesDecodeFail() {
+        EmbeddedChannel e = new EmbeddedChannel(new Socks5InitialRequestDecoder());
+        assertFalse(e.writeInbound(Unpooled.wrappedBuffer(new byte[]{5, 2, 0})));
+        assertTrue(e.writeInbound(Unpooled.wrappedBuffer(new byte[]{1})));
+        Object o = e.readInbound();
+
+        assertTrue(o instanceof DefaultSocks5InitialRequest);
+        DefaultSocks5InitialRequest req = (DefaultSocks5InitialRequest) o;
+        assertSame(req.decoderResult(), DecoderResult.SUCCESS);
+        assertFalse(e.finish());
+    }
+}