Also make printing of an incomplete AST more robust.

Change-Id: I56636890deb6e38882a3f8206aff7cde3e4bab60
Reviewed-on: https://chromium-review.googlesource.com/1168498Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55009}