In a fuzzer testcase I found locally the ast-decoder spent a lot of time
in the decoding of kExprBrTable after the decoder already found an
error. I did not add a test because the fuzzer test case did not
actually crash. It only run out of memory in the chromium fuzzer.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2413863003
Cr-Commit-Position: refs/heads/master@{#40254}