midx: don't free midx_name early

A subsequent patch will need to refer back to 'midx_name' later on in
the function. In fact, this variable is already free()'d later on, so
this makes the later free() no longer redundant.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -956,7 +956,6 @@ static int write_midx_internal(const char *object_dir, struct multi_pack_index *
 
 	hold_lock_file_for_update(&lk, midx_name, LOCK_DIE_ON_ERROR);
 	f = hashfd(get_lock_file_fd(&lk), get_lock_file_path(&lk));
-	FREE_AND_NULL(midx_name);
 
 	if (ctx.m)
 		close_midx(ctx.m);