slider: fix segfault in case of obj is deleted before wheel timeout timer expires
slider: fix segfault in case of obj is deleted before wheel timeout timer expires

Issue:
1. elementary_test -> slider
2. move mousewheel on slider and popup gets shown.
3. close the slider window very fast or increase the wheel_timeout to 4 seconds
4. observe crash

tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary