elm/scrollable: Avoid division by zero during scroll.

elm/scrollable: Avoid division by zero during scroll.

When several mouse move events arrive with the same timestamps, the
calculation done to get an average of movement will try to divide things
by zero. This fix will make it ignore this calculation and just go back
to assuming that there's only one last move event.

Such behavior can happen on wayland backend, when using the X11 backend
for Weston, since it will process several X events in one loop iteration
and send all of them with the same timestamp.