I had a segfault on OS X today at Pterm.app shutdown. I wasn't able to
reproduce it in a debugger, but the cause seemed to be that
clipboard_clear called term_deselect (this was from before the patch
series that renamed that function) when inst->term was already NULL.
This must be because a clipboard_data_instance outlived its associated
inst->term, and quite likely its associated inst as well. But we can't
free those structures when a gui_data is freed, because GTK callbacks
will still depend on them; so instead we must have each gui_data keep
a list of active cdis pointing at it, and then at destruction time,
walk along the list nulling out each one's pointer to part of itself.
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