bisect: handle NULL commit in `bisect_successful()`

bisect: handle NULL commit in `bisect_successful()`

When `lookup_commit_reference_by_name()` is called to find the first bad
commit, the result is passed to `repo_format_commit_message()`
immediately, which dereferences commit without checking for NULL.

However, the commit could be NULL, even though in practice this is
unlikely because `bisect_successful()` is only called after a successful
bisect run has identified the bad commit, but the ref could still become
dangling due to a concurrent gc or repository corruption.

Pointed out by Coverity.

Assisted-by: Claude Opus 4.6
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>