Handle partially sent varints better

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

@@ -20,7 +20,8 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
         return;
       }
 
-      if (!reader.successfulDecode) {
+      if (reader.successfulDecode) {
+        if (reader.readVarint < 0) {
           throw BAD_LENGTH_CACHED;
         }
 
@@ -33,6 +34,13 @@ public class MinecraftVarintFrameDecoder extends ByteToMessageDecoder {
           reader.reset();
           return;
         }
+      } else {
+        boolean tooBig = reader.bytesRead > 3;
+        reader.reset();
+        if (tooBig) {
+          throw BAD_LENGTH_CACHED;
+        }
+      }
     }
   }