Do not crash on null stream in writing to bidirectional streams

net/http/bidirectional_stream_impl.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_HTTP_BIDIRECTIONAL_STREAM_IMPL_H_
 #define NET_HTTP_BIDIRECTIONAL_STREAM_IMPL_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 56 matching lines @@
     // The delegate should not call BidirectionalStreamImpl::Cancel
     // during this callback.
     virtual void OnDataSent() = 0;
 
     // Called when trailers are received. This is called as soon as trailers
     // are received, which can happen before a read completes.
     // The delegate is able to continue reading if there is no pending read and
     // EOF has not been received, or to send data if there is no pending send.
     virtual void OnTrailersReceived(const SpdyHeaderBlock& trailers) = 0;
 
-    // Called when an error occurred.
-    // No other delegate functions will be called after this.
+    // Called when an error occurred. Do not call into the stream after this
+    // point. No other delegate functions will be called after this.
     virtual void OnFailed(int status) = 0;
 
    protected:
     virtual ~Delegate();
 
    private:
     DISALLOW_COPY_AND_ASSIGN(Delegate);
   };
 
   BidirectionalStreamImpl();
@@ skipping 64 matching lines @@
   // not associated with any stream, and are not included in this value.
   virtual int64_t GetTotalSentBytes() const = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(BidirectionalStreamImpl);
 };
 
 }  // namespace net
 
 #endif  // NET_HTTP_BIDIRECTIONAL_STREAM_IMPL_H_