bisect--helper: avoid use-after-free

bisect--helper: avoid use-after-free

In 5e82c3dd22a (bisect--helper: `bisect_reset` shell function in C,
2019-01-02), the `git bisect reset` subcommand was ported to C. When the
call to `git checkout` failed, an error message was reported to the
user.

However, this error message used the `strbuf` that had just been
released already. Let's switch that around: first use it, then release
it.

Mentored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>