The fuzzer found a couple of cases that exploited comments of the
form:

  function test() {
    const re = /*.../;
    const str = '...*/...';
    let result;
    try { result = re.exec(str); } catch (e) { /* ... */ }
  }

Note that the first line does not contain a regexp literal, it starts
a comment instead. The second line terminates the comment.

This fixes detection of such cases by initializing `result` to null.

TBR=yangguo@chromium.org

Bug: chromium:805970
Change-Id: I5d46db9892e2b4e71cdc2907cebf07a2e33b7a0e
Reviewed-on: https://chromium-review.googlesource.com/894403Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50991}