Fix a regression introduced in 4ce6d10601
which causes devices with an odd-numbered zero-based index in the list
to be passed over incorrectly. This might present as yet another "device
does not send pressure" bug for ~50% of devices out there.
This commit also closes off another potential segfault for wintab_devices
lists which have an odd length.
https://bugzilla.gnome.org/show_bug.cgi?id=774699
