label: set sliding only if label size is valid

label: set sliding only if label size is valid

Summary:
When call slide_go before label is resized, setting the sliding is a meaningless.
This patch is for reducing redundant operations.

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Deepjyoti Dutta <deepjyoti.d@samsung.com>
Test Plan: elementary_test -to "label slide"

Reviewers: cedric, Hermet

Subscribers: seoz, jpeg

Differential Revision: https://phab.enlightenment.org/D4089

Signed-off-by: Cedric Bail <cedric@osg.samsung.com>