mailinfo: fix leaking header data

mailinfo: fix leaking header data

We populate the `mailinfo` arrays `p_hdr_data` and `s_hdr_data` with
data parsed from the mail headers. These arrays may end up being only
partially populated with gaps in case some of the headers do not parse
properly. This causes memory leaks because `strbuf_list_free()` will
stop iterating once it hits the first `NULL` pointer in the backing
array.

Fix this by open-coding a variant of `strbuf_list_free()` that knows to
iterate through all headers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>