util.h: correct range check of str_to_int_gt() and str_to_int_lt()

The range checks of str_to_int_gt() and str_to_int_lt() are not
implemented literally.  The current definitions fail to reject a
marginal value.

Due to this difference, tgtd accepts option "-t 0" without an error,
which overrides nr_iothreads to 0 and causes clients to hang.

This corrects str_to_int_gt()/str_to_int_lt() and prevents the issue.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>