Fix a crash in the FileStream::Context::Read code path where we were invoking a NULL callback.

net/base/file_stream_context_win.cc

Index: net/base/file_stream_context_win.cc
diff --git a/net/base/file_stream_context_win.cc b/net/base/file_stream_context_win.cc
index 5ffbde72388293a339e46430846f4be6df62695c..4e70f69abd8a5b48fca0d2e8fd5cf1ed8bcf12e5 100644
--- a/net/base/file_stream_context_win.cc
+++ b/net/base/file_stream_context_win.cc
@@ -174,6 +174,8 @@ void FileStream::Context::OnIOCompleted(
 }
 
 void FileStream::Context::InvokeUserCallback() {
+  if (callback_.is_null())
+    return;
   // For an asynchonous Read operation don't invoke the user callback until
   // we receive the IO completion notification and the asynchronous Read
   // completion notification.