listbox: Store child iter in a variable when removing

listbox: Store child iter in a variable when removing

Unparenting a GtkListBoxRow can drop its last reference, which
will free its memory. Right after unparenting, though, we were
accessing the row's iter - which assumes that the row is still
alive. This causes a crash when, for example, binding two or
more models to the listbox.

Fix that by storing the iter in a variable, and not trying to
access it after unparenting. After unparenting, the variables
that are potentially garbage were explicitly assigned NULL for
clarity.

Fixes https://gitlab.gnome.org/GNOME/gtk/issues/1258