elementary: Check for valid resize object

Summary:
Fix an issue where elm_slider was passing NULL to eo functions due to
wd->resize_obj being NULL. Discovered via Enlightenment mixer popup
dialog

ref T7030
Depends on D6639

Reviewers: zmike

Reviewed By: zmike

Subscribers: Hermet, bu5hm4n, cedric, committers

Tags: efl

Maniphest Tasks: T7030

Differential Revision: https://phab.enlightenment.org/D6640