sha1-name: do not assume that the ref store is initialized

c931ba4e (sha1-name.c: remove the_repo from handle_one_ref(),
2019-04-16) replaced the use of for_each_ref() helper, which works
with the main ref store of the default repository instance, with
refs_for_each_ref(), which can work on any ref store instance, by
assuming that the repository instance the function is given has its
ref store already initialized.

But it is possible that nobody has initialized it, in which case,
the code ends up dereferencing a NULL pointer.

Reported-by: Érico Rolim <erico.erc@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1-name.c

@@ -1771,7 +1771,7 @@ static enum get_oid_result get_oid_with_context_1(struct repository *repo,
 
 			cb.repo = repo;
 			cb.list = &list;
-			refs_for_each_ref(repo->refs, handle_one_ref, &cb);
+			refs_for_each_ref(get_main_ref_store(repo), handle_one_ref, &cb);
 			refs_head_ref(repo->refs, handle_one_ref, &cb);
 			commit_list_sort_by_date(&list);
 			return get_oid_oneline(repo, name + 2, oid, list);

t/t4208-log-magic-pathspec.sh

@@ -55,6 +55,10 @@ test_expect_success '"git log -- :/a" should not be ambiguous' '
 	git log -- :/a
 '
 
+test_expect_success '"git log :/any/path/" should not segfault' '
+	test_must_fail git log :/any/path/
+'
+
 # This differs from the ":/a" check above in that :/in looks like a pathspec,
 # but doesn't match an actual file.
 test_expect_success '"git log :/in" should not be ambiguous' '