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 9c1e657a8f (fetch: teach independent negotiation (no
packfile), 2021-05-04).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5702-protocol-v2.sh

@@ -599,6 +599,22 @@ setup_negotiate_only () {
 	test_commit -C client three
 }
 
+test_expect_success 'usage: --negotiate-only without --negotiation-tip' '
+	SERVER="server" &&
+	URI="file://$(pwd)/server" &&
+
+	setup_negotiate_only "$SERVER" "$URI" &&
+
+	cat >err.expect <<-\EOF &&
+	fatal: --negotiate-only needs one or more --negotiate-tip=*
+	EOF
+
+	test_must_fail git -c protocol.version=2 -C client fetch \
+		--negotiate-only \
+		origin 2>err.actual &&
+	test_cmp err.expect err.actual
+'
+
 test_expect_success 'file:// --negotiate-only' '
 	SERVER="server" &&
 	URI="file://$(pwd)/server" &&