efreet: check magic only for file which has size

efreet: check magic only for file which has size

Summary:
Due to unnecessary magic checking, there was freezing in /proc in efm.
Proc file's st_size is zero, but, it's readable. therfore, it takes unnecessary time in magic checking. And, there is no need to check magic in case of 0 sized regular files as well.

Therefore, skip magic check in case of st_size is zero.

Fixes T1173

Test Plan: enlightenment -> file browser (efm) -> get int /proc --> check whether efm freezes or not

Reviewers: raster, cedric, zmike

CC: seoz, cedric
Maniphest Tasks: T1173

Differential Revision: https://phab.enlightenment.org/D764

Signed-off-by: Cedric Bail <cedric.bail@free.fr>