strtoul_ui: reject negative values

strtoul_ui: reject negative values

strtoul_ui uses strtoul to get a long unsigned, then checks that casting
to unsigned does not lose information and return the casted value.

On 64 bits architecture, checking that the cast does not change the value
catches most errors, but when sizeof(int) == sizeof(long) (e.g. i386),
the check does nothing. Unfortunately, strtoul silently accepts negative
values, and as a result strtoul_ui("-1", ...) raised no error.

This patch catches negative values before it's too late, i.e. before
calling strtoul.

Reported-by: Max Kirillov <max@max630.net>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>