[PATCH] bisect: avoid NULL pointer dereference

["René Scharfe" <l.s.r@web.de>]

7c117184d7 (bisect: fix off-by-one error in `best_bisection_sorted()`)
fixed an off-by-one error, plugged a memory leak and removed a NULL
check.  However, the pointer p *is* actually NULL if an empty list is
passed to the function.  Let's add the check back for safety.  Bisecting
nothing doesn't make too much sense, but that's no excuse for crashing.

Found with GCC's -Wnull-dereference.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
---
 bisect.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/bisect.c b/bisect.c
index 0fca17c02b..2f3008b078 100644
--- a/bisect.c
+++ b/bisect.c
@@ -229,8 +229,10 @@ static struct commit_list *best_bisection_sorted(struct commit_list *list, int n
 		if (i < cnt - 1)
 			p = p->next;
 	}
-	free_commit_list(p->next);
-	p->next = NULL;
+	if (p) {
+		free_commit_list(p->next);
+		p->next = NULL;
+	}
 	strbuf_release(&buf);
 	free(array);
 	return list;
-- 
2.15.1