spinner: crash on illegal format set issue fix.
spinner: crash on illegal format set issue fix.

Summary:
Set any illegal format, spinner crashes, the format set to spinner
has to be valid hence a check is added initially itself to check for validity of
label formats.

Signed-off-by: Shilpa Singh <shilpa.singh@samsung.com>

Test Plan:
1. Set illegal format to spinner for e.g: elm_spinner_label_format_set(sp, "d");
2. Run spinner demo
Crash is observed

Reviewers: cedric

Subscribers: govi, buds

Differential Revision: https://phab.enlightenment.org/D3668

Signed-off-by: Cedric BAIL <240633aa59d25638de9800ef43a88ad2e208d24d@osg.samsung.com>

tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary,tasn/elementary