controller: Fix gst_interpolation_control_source_find_control_point_iter

controller: Fix gst_interpolation_control_source_find_control_point_iter

The logic in that function is broken. Various NULL-checking bandaids for
guaranteed non-NULL variables didn't even help there.

This patch updates the function to check if a previous item exists
before fetching it instead of after. This makes all other tests
unnecessary.
In particular, it makes the check for an empty list unnecessary, because
for empty lists the only iter is the begin iter (and the end iter) and
so the new check catches that case.

https://bugzilla.gnome.org/show_bug.cgi?id=616846