build: fix warning re comparison of enum and uint

The warning is observed when tarantool is compiled by GCC 9.1.0.

Warnings are treated as errors during a debug build or when
-DENABLE_WERROR=ON option is passed to cmake, that is usual for our
testing jobs in CI.

The commit that introduces the problem is
3a8adccf ('access: fix invalid error
type for not found user').

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>