ecore-wl2: Clarify post-condition for global lookup in _ecore_wl2_shell_bind

ecore-wl2: Clarify post-condition for global lookup in _ecore_wl2_shell_bind

Summary:
The !global test is a post-condition that _ecore_wl2_global_find
provided valid data, so the code is clearer if this is kept separate
from the subsequent if chain for looking up the shell.  Since this
post-condition returns from the function on error, it can stand alone.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewers: devilhorns, ManMower

Reviewed By: ManMower

Subscribers: jpeg, cedric

Differential Revision: https://phab.enlightenment.org/D4690