Wrong error when inserting LyX file

[Jean-Marc Lasgouttes]

When inserting a LyX file, I get the (harmless) error
  Error in LyXLex::setStream: file or stream already set.

The problem is that BufferView::insertLyXFile opens the file to see
whether it begins with '#', so that LyXLex thinks the streams is not
good to use.

Maybe the test should be changed, but I do not know how.

JMarc