Patch for Export Bug

[Richard Kimberly Heck]

There's a patch for a nasty export bug here:

     https://www.lyx.org/trac/ticket/13017

I thought it best not to commit it yet, but I'd really like to solve 
this before 2.4.0. I think we'll get reports if we don't. From a testing 
point of view, I'm less interested in whether it fixes that bug than in 
whether it causes other problems. I don't think it should, but...

Riki