efl_ui_tab_bar: fix error about double free

Summary:
tab is the parent of the icon.
so deleting the tab causes deleting the icon automatically.
in this case, it can cause double-freed problem once icon has deleted.

Test Plan: elementary_test -to efl.ui.tab_pager

Reviewers: cedric, Jaehyun_Cho, jpeg

Reviewed By: Jaehyun_Cho

Subscribers: reviewers, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D7407