Fix bug discovered by Kornel. See

Fix bug discovered by Kornel. See
	http://marc.info/?l=lyx-devel&m=138590578911716&w=2
If you look at Buffer.cpp, around line 4351, there was a comment about bug 5699. We are seeing the
same crash. The problem is that, although the master does have a GUI, that GUI is in a different window. So the structureChanged() call we do during updateBuffer() is for the TOC in that window, not the TOC in the window we are actually in. So our TocModel::toc_ has been reset and is invalid, though the widget itself has not been updated and looks fine.

This patch tests whether the master is in the same window as the buffer we are updating.

A problem remains, which is noted in a comment.