This fixes a simple error in the parser fuzzer, where we accessed the
{parentheses} vector without checking that it is non-empty.

Drive-by: Some formatting cleanup, and switch to {vector} for
  performance.

R=mstarzinger@chromium.org

Bug: chromium:1027132
Change-Id: I5faa39885801953f2fb698b1131eab1f138a524d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1936472Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65179}