efl_ui_exact_model: use eina APIs to get the nth data

Summary:
Also, fixes a compiler warning

../src/lib/elementary/efl_ui_exact_model.c:60:17: warning: variable 'tbuf' set but not used [-Wunused-but-set-variable]

Test Plan: ninja

Reviewers: cedric, zmike, bu5hm4n

Reviewed By: zmike

Subscribers: reviewers, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D11380