Possible segfault introduced in commit.c

Hi all,

As you might already know, we analyze git regularly with Sentry (our static analysis tool). Today it picked up a new NULL pointer dereference in commit.c:366:

    void commit_list_reverse(struct commit_list **list_p)
    {
        struct commit_list *prev = NULL, *curr = *list_p, *next;

        if (!list_p)
            return;
        /* function continues... */
    }

list_p is dereferenced on the first line, then tested for NULL on the very next statement. If it's possible that list_p is NULL, this will be a segfault. If it can't be NULL, then the check is unnecessary (and probably misleading).

Introduced here: https://github.com/gitster/git/commit/fbc08ea

Best, Mike