swipeTracker: Handle invalid position when finding previous point

swipeTracker: Handle invalid position when finding previous point

Commit 8e29babca8adc677 switched to `findLastIndex()` to find
the previous point, which inadvertently changed what value is
returned for invalid input.

Restore the old fallback value and log a stacktrace, so we can
hopefully track down what causes the invalid input.

Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8856
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/4062>
(cherry picked from commit eb9caf0028be0d5740fc419e7e865584c0f52cab)