ecore xi2: Discard "axis" inputs from standard mice

ecore xi2: Discard "axis" inputs from standard mice

Mice in X with xi2 send Axis events which are badly defined,
and carry basically useless information, as we also receive
proper mouse events. Notably, all mice input events are
"Rel something" but in fact they are absolute values (even
the wheel information is a counter increasing every time you
scroll).

This should not break any application as such axis events
carried only values with label ECORE_AXIS_LABEL_UNKNOWN.
This also fixes a leak when n == 0 (no "valuator" found
in the list, this used to be unlikely, now happens at every
mouse event).