crbug.com/518788 looks like AstValueFactory::strings_ would contain
nullptrs which shouldn't be possible. This CL makes us crash earlier
before putting nullptrs into it. It's paranoid, since Zone should
already crash before returning nullptr.

BUG=518788
LOG=n
R=adamk@chromium.org

Review-Url: https://codereview.chromium.org/2156513002
Cr-Commit-Position: refs/heads/master@{#37817}