Only call ToString in fuzzer if IsSetConfig.

ToString (which is a test-only method so it doesn't really matter) is
only call if IsSetConfig, not !IsParseError

Bug: 954766
Change-Id: I1e956cb374d5bf9899e16942f6c13868e29e6100
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1577563
Commit-Queue: David Benjamin <davidben@chromium.org>
Auto-Submit: David Benjamin <davidben@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652906}
3 files changed