git-mailinfo: Fix getting the subject from the in-body [PATCH] line

git-mailinfo: Fix getting the subject from the in-body [PATCH] line

"Subject: " isn't in the static array "header", and thus
memcmp("Subject:", header[i], 7) will never match.

Even if it did so, hdr_data[] may not have been allocated if there weren't
a "Subject: " in-body when we process "[PATCH]" in the affected codepath.

Signed-off-by: Lukas Sandström <lukass@etek.chalmers.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>