builtin/multi-pack-index.c: don't leak concatenated options

builtin/multi-pack-index.c: don't leak concatenated options

The `multi-pack-index` builtin dynamically allocates an array of
command-line option for each of its separate modes by calling
add_common_options() to concatante the common options with sub-command
specific ones.

Because this operation allocates a new array, we have to be careful to
remember to free it. We already do this in the repack and write
sub-commands, but verify and expire don't. Rectify this by calling
FREE_AND_NULL as the other modes do.

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