reachable.c::add_one_tree: handle NULL from lookup_tree

Signed-off-by: Martin Koegler <mkoegler@auto.tuwien.ac.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reachable.c

@@ -150,6 +150,7 @@ static int add_one_reflog(const char *path, const unsigned char *sha1, int flag,
 static void add_one_tree(const unsigned char *sha1, struct rev_info *revs)
 {
 	struct tree *tree = lookup_tree(sha1);
+	if (tree)
 		add_pending_object(revs, &tree->object, "");
 }