sparse-index.c: expand_to_path() can assume non-NULL "istate"

sparse-index.c: expand_to_path() can assume non-NULL "istate"

This function added in [1] was subsequently used in [2]. All of the
calls to it are in name-hash.c, and come after calls to
lazy_init_name_hash(istate). The first thing that function does is:

	if (istate->name_hash_initialized)
		return;

So we can already assume that we have a non-NULL "istate" here, or
we'd be segfaulting. Let's not confuse matters by making it appear
that's not the case.

1. 71f82d032f3 (sparse-index: expand_to_path(), 2021-04-12)
2. 4589bca829a (name-hash: use expand_to_path(), 2021-04-12)

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