Do not drain HttpContentReadStream if the connection is disposed (#57287)

Fixes #56159.

Author: antonfirsov

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ChunkedEncodingReadStream.cs

@@ -425,7 +425,7 @@ private enum ParsingState : byte
                 Done
             }
 
-            public override bool NeedsDrain => (_connection != null);
+            public override bool NeedsDrain => CanReadFromConnection;
 
             public override async ValueTask<bool> DrainAsync(int maxDrainBytes)
             {

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/ContentLengthReadStream.cs

@@ -190,7 +190,7 @@ private ReadOnlyMemory<byte> ReadFromConnectionBuffer(int maxBytesToRead)
                 return connectionBuffer.Slice(0, bytesToConsume);
             }
 
-            public override bool NeedsDrain => (_connection != null);
+            public override bool NeedsDrain => CanReadFromConnection;
 
             public override async ValueTask<bool> DrainAsync(int maxDrainBytes)
             {

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpContentReadStream.cs

@@ -28,6 +28,17 @@ public HttpContentReadStream(HttpConnection connection) : base(connection)
 
             protected bool IsDisposed => _disposed == 1;
 
+            protected bool CanReadFromConnection
+            {
+                get
+                {
+                    // _connection == null typically means that we have finished reading the response.
+                    // Cancellation may lead to a state where a disposed _connection is not null.
+                    HttpConnection? connection = _connection;
+                    return connection != null && connection._disposed != Status_Disposed;
+                }
+            }
+
             public virtual ValueTask<bool> DrainAsync(int maxDrainBytes)
             {
                 Debug.Fail($"DrainAsync should not be called for this response stream: {GetType()}");