midx: don't peek into `struct lock_file`

Similar to the previous commits, avoid peeking into the `struct
lock_file`. Use the lock file API instead.

The two functions we're calling here double-check that the tempfile is
indeed "active", which is arguably overkill considering how we took the
lock on the line immediately above. More importantly, this future-proofs
us against, e.g., other code appearing between these two lines or the
lock file and/or tempfile internals changing.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -918,7 +918,7 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 					(pack_name_concat_len % MIDX_CHUNK_ALIGNMENT);
 
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
-	f = hashfd(lk.tempfile->fd, lk.tempfile->filename.buf);
+	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
 	FREE_AND_NULL(midx_name);
 
 	if (packs.m)