tree-diff: don't access hash of NULL object_id pointer

The object_id pointers can be NULL for invalid entries.  Don't try to
dereference them and pass NULL along to fill_tree_descriptor() instead,
which handles them just fine.

Found with Clang's UBSan.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tree-diff.c

@@ -422,8 +422,9 @@ static struct combine_diff_path *ll_diff_tree_paths(
 	 *   diff_tree_oid(parent, commit) )
 	 */
 	for (i = 0; i < nparent; ++i)
-		tptree[i] = fill_tree_descriptor(&tp[i], parents_oid[i]->hash);
-	ttree = fill_tree_descriptor(&t, oid->hash);
+		tptree[i] = fill_tree_descriptor(&tp[i],
+				parents_oid[i] ? parents_oid[i]->hash : NULL);
+	ttree = fill_tree_descriptor(&t, oid ? oid->hash : NULL);
 
 	/* Enable recursion indefinitely */
 	opt->pathspec.recursive = DIFF_OPT_TST(opt, RECURSIVE);