Lists of points come in coordinate pairs, but we didn't have any checking for that.
It was possible to try to fetch the 'last' coordinate in a list, i.e. the y coordinate
of an x,y pair, that was in fact missing, leading to an out-of-bounds array read.

In that case, we now reuse the last-known y coordinate.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=738050

Signed-off-by: Federico Mena Quintero <federico@gnome.org>