fetch: fix segfault in --negotiate-only without --negotiation-tip=*

fetch: fix segfault in --negotiate-only without --negotiation-tip=*

The recent --negotiate-only option would segfault in the call to
oid_array_for_each() in negotiate_using_fetch() unless one or more
--negotiation-tip=* options were provided.

All of the other tests for the feature combine both, but nothing was
checking this assumption, let's do that and add a test for it. Fixes a
bug in 9c1e657a8fd (fetch: teach independent negotiation (no
packfile), 2021-05-04).

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>