lib/checkout: Validate pathnames during checkout

While we do protect against path traversal during pull, let's also validate
during checkout; it's a cheap operation and provides good last-mile protection.

Closes: #1412
Approved by: jlebon

tests/test-corruption.sh

@@ -19,7 +19,7 @@
 
 set -euo pipefail
 
-echo "1..5"
+echo "1..6"
 
 . $(dirname $0)/libtest.sh
 
@@ -79,6 +79,11 @@ if ${CMD_PREFIX} ostree --repo=ostree-path-traverse/repo fsck -q 2>err.txt; then
     fatal "fsck unexpectedly succeeded"
 fi
 assert_file_has_content_literal err.txt '.dirtree: Invalid / in filename ../afile'
+echo "ok path traverse fsck"
 
-echo "ok path traverse"
-
+cd ${test_tmpdir}
+if ${CMD_PREFIX} ostree --repo=ostree-path-traverse/repo checkout pathtraverse-test pathtraverse-test 2>err.txt; then
+    fatal "checkout with path traversal unexpectedly succeeded"
+fi
+assert_file_has_content_literal err.txt 'Invalid / in filename ../afile'
+echo "ok path traverse checkout"