Delete faulty assert in SslStreamInternal (#29131)

We're hitting an assert when a ReadAsync call is made on SslStream after the underlying stream encounters EOF.  When calling FillBufferAsync, we've already allocated the read buffer, and we may not have any data in the buffer; if we then hit EOF, we've still not got any data in the buffer and we return.  Then when ReadAsync is called again, the assert trips over the buffer being allocated and also being empty.

This just deletes the assert.  We could also return the buffer when we hit EOF, but that'll be done by Dispose, anyway.

Author: stephentoub

src/System.Net.Security/src/System/Net/Security/SslStreamInternal.cs

@@ -182,7 +182,6 @@ internal Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken
         private void ResetReadBuffer()
         {
             Debug.Assert(_decryptedBytesCount == 0);
-            Debug.Assert(_internalBuffer == null || _internalBufferCount > 0);
 
             if (_internalBuffer == null)
             {