THRIFT-5480 TThreadPoolAsyncServer using TFramedTransport mistakenly drops client
Client: netstd
Patch:  Ioannis Efthymiou & Jens Geyer

This closes #2478
diff --git a/lib/netstd/Thrift/Transport/Layered/TFramedTransport.cs b/lib/netstd/Thrift/Transport/Layered/TFramedTransport.cs
index be1513f..f560986 100644
--- a/lib/netstd/Thrift/Transport/Layered/TFramedTransport.cs
+++ b/lib/netstd/Thrift/Transport/Layered/TFramedTransport.cs
@@ -86,6 +86,7 @@
 
         private async ValueTask ReadFrameAsync(CancellationToken cancellationToken)
         {
+            UpdateKnownMessageSize(-1);
             await InnerTransport.ReadAllAsync(HeaderBuf, 0, HeaderSize, cancellationToken);
             int size = BinaryPrimitives.ReadInt32BigEndian(HeaderBuf);