The call to gtk_list_store_clear() in unifontsel_setup_familylist()
was causing a call to family_changed() via the GTK signal system,
which didn't happen in GTK2. family_changed() in turn was calling
unifontsel_select_font(), which got confused when the tree model
didn't match reality, and tried to access a bogus tree iterator.
This is easily fixed by using the existing fs->inhibit_response flag,
which prevents us responding to GTK events when we know they were
generated by our own fiddling about with the data; it's just that we
never needed to set it in unifontsel_setup_familylist() before.
Also, added a check of the return value from the key get_iter call in
unifontsel_select_font(), so that it'll at least fail an assertion
rather than actually trying to access bogus memory. But that operation
_should_ still always succeed, and if it doesn't, it's probably a sign
that we need another use of fs->inhibit_response.