Jehan authored May 14, 2018 and Michael Natterer committed May 20, 2018

This fixes a crash when we were looping until GDK_AXIS_LAST, but our
existing code was expecting less values.

Also I note that GTK+3 has an additional GDK_AXIS_ROTATION since version
3.22, even though we were told that some pens with rotation support
would report rotation values with same valuator as for the wheel (at
least, the Wacom Art Pen would apparently use the same 6th valuator).
For proper support, it would be really worth testing such pens.