Memory problem in Rgui script editor? (PR#7360)

I've seen warnings from gdb that something is reading from memory that
has already been free'd when the script editor is closed in Rgui.  

I haven't seen any bad symptoms of this outside the debugger, so I may
not take the time to track it down and fix it before the 2.0.1
release:  but I don't want to forget about it, hence this bug report.

Duncan Murdoch