Non-printable characters do not make sense.
Inputs with non balanced brackets are mostly useless as well.

This validation function makes the fuzzer 15-20x faster.

Also use -only_ascii=1 option of libFuzzer:
https://codereview.chromium.org/2875933003

BUG=chromium:584819

Review-Url: https://codereview.chromium.org/2881583002
Cr-Commit-Position: refs/heads/master@{#45367}