Improve reliability of varint decoder.

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftVarintFrameDecoder.java

@@ -2,8 +2,10 @@ package com.velocitypowered.proxy.protocol.netty;
 
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.handler.codec.ByteToMessageDecoder;
+import io.netty.handler.codec.CorruptedFrameException;
 
 import java.util.List;
 
@@ -15,12 +17,26 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
         }
 
         in.markReaderIndex();
-        int packetLength = ProtocolUtils.readVarInt(in);
-        if (in.readableBytes() < packetLength) {
-            in.resetReaderIndex();
-            return;
+
+        byte[] lenBuf = new byte[3];
+        for (int i = 0; i < lenBuf.length; i++) {
+            lenBuf[i] = in.readByte();
+            if (lenBuf[i] > 0) {
+                int packetLength = ProtocolUtils.readVarInt(Unpooled.wrappedBuffer(lenBuf));
+                if (packetLength == 0) {
+                    return;
+                }
+
+                if (in.readableBytes() < packetLength) {
+                    in.resetReaderIndex();
+                    return;
+                }
+
+                out.add(in.readRetainedSlice(packetLength));
+                return;
+            }
         }
 
-        out.add(in.readRetainedSlice(packetLength));
+        throw new CorruptedFrameException("VarInt too big");
     }
 }