Wheel event widget: Harden logic an extra bit

This is quite an unlikely scenario, but not impossible.

It could be that the wheel widget is destroyed during
an update phase event. In that case, wheel_widget would
be a dangling pointer for any subsequent wheel event.
We protect against this with a QPointer.

However, that would mean that if the next wheel event
were to be an end phase event, that event would be lost.
So we go through the usual code path, except that we won't
set wheel_widget in the case of an end phase event.

Change-Id: I59a912b845dcc249e1edc60b4dc28bf308d807d9
Reviewed-by: Shawn Rutledge <shawn.rutledge@theqtcompany.com>