midx.c: don't leak MIDX from verify_midx_file

midx.c: don't leak MIDX from verify_midx_file

The function midx.c:verify_midx_file() allocates a MIDX struct by
calling load_multi_pack_index(). But when cleaning up, it calls free()
without freeing any resources associated with the MIDX.

Call the more appropriate close_midx() which does free those resources,
which causes t5319.3 to pass when Git is compiled with SANITIZE=leak.

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