busybox

dhcpd: fix an improper widening conversion

We wanted to detect when tv_sec = unsigned1 - unsigned2
underflows by looking at tv_sec's sign. But if tv_sec
is long and it is wider than unsigned, we get unsigned -> long
conversion which is in this case never negative.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>