[d8] bounds-check before getting Shell::Worker internal field

Prevents fatal error in debug builds

BUG=v8:4271, 506954
R=binji@chromium.org
LOG=N

Committed: https://crrev.com/43ce9c6f101c4224addd9a54e0c39963188dc7fa
Cr-Commit-Position: refs/heads/master@{#29524}

Review URL: https://codereview.chromium.org/1214053004

Cr-Commit-Position: refs/heads/master@{#29737}