e_comp_wl: fix segfault in case parent's surface is destroyed before subsurface is...

e_comp_wl: fix segfault in case parent's surface is destroyed before subsurface is destroyed

Summary:
A parent surface can be destroyed. In this case, ec->comp_data->sub.data->parent has
a wrong address which has already destroyed. Then, it occurs segmentation fault.
To avoid segfault, ec->comp_data->sub.data->parent should be NULL when a parent surface
is destroyed.

Signed-off-by: Boram Park <boram1288.park@samsung.com>
Test Plan: Run attached test program(subsurface)

Reviewers: gwanglim, cedric, devilhorns, zmike

Reviewed By: devilhorns, zmike

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D2484