efl_check: unset the callback here
this is needed in order to not accidently overwrite stack values.
This never showed up while running the test suite, as erroring would
have aborted anyways. However, when we are running with CK_FORK=no, this
may leak into another test.
Reviewed-by: Mike Blumenkrantz <email@example.com>
Differential Revision: https://phab.enlightenment.org/D11595