list-objects.c: handle unexpected non-tree entries

Apply similar treatment as the previous commit for non-tree entries,
too.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects.c

@@ -125,6 +125,11 @@ static void process_tree_contents(struct traversal_context *ctx,
 
 		if (S_ISDIR(entry.mode)) {
 			struct tree *t = lookup_tree(ctx->revs->repo, &entry.oid);
+			if (!t) {
+				die(_("entry '%s' in tree %s has tree mode, "
+				      "but is not a tree"),
+				    entry.path, oid_to_hex(&tree->object.oid));
+			}
 			t->object.flags |= NOT_USER_GIVEN;
 			process_tree(ctx, t, base, entry.path);
 		}

t/t6102-rev-list-unexpected-objects.sh

@@ -34,8 +34,9 @@ test_expect_failure 'traverse unexpected non-tree entry (lone)' '
 	test_must_fail git rev-list --objects $broken_tree
 '
 
-test_expect_failure 'traverse unexpected non-tree entry (seen)' '
-	test_must_fail git rev-list --objects $blob $broken_tree
+test_expect_success 'traverse unexpected non-tree entry (seen)' '
+	test_must_fail git rev-list --objects $blob $broken_tree >output 2>&1 &&
+	test_i18ngrep "is not a tree" output
 '
 
 test_expect_success 'setup unexpected non-commit parent' '