V8 doesn't build on Ubuntu 16.04 (with GCC 5.3). Seems to be
a known regression on newer GCC version. It emits incorrect
"error: array subscript is above array bounds" message. Adding
explicit array bound check fixes the issue.

R=hablich@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2256113002
Cr-Commit-Position: refs/heads/master@{#38721}