sequencer.c: always free() the "msgbuf" in do_pick_commit()

sequencer.c: always free() the "msgbuf" in do_pick_commit()

In [1] the strbuf_release(&msgbuf) was moved into this
do_pick_commit(), but didn't take into account the case of [2], where
we'd return before the strbuf_release(&msgbuf).

Then when the "fixup" support was added in [3] this leak got worse, as
in this error case we added another place where we'd "return" before
reaching the strbuf_release().

This changes the behavior so that we'll call
update_abort_safety_file() in these cases where we'd previously
"return", but as noted in [4] "update_abort_safety_file() is a no-op
when rebasing and you're changing code that is only run when
rebasing.". Here "no-op" refers to the early return in
update_abort_safety_file() if git_path_seq_dir() doesn't exist.

1. 452202c74b8 (sequencer: stop releasing the strbuf in
   write_message(), 2016-10-21)
2. f241ff0d0a9 (prepare the builtins for a libified merge_recursive(),
   2016-07-26)
3. 6e98de72c03 (sequencer (rebase -i): add support for the 'fixup' and
   'squash' commands, 2017-01-02)
4. https://lore.kernel.org/git/bcace50b-a4c3-c468-94a3-4fe0c62b3671@dunelm.org.uk/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>