Short external strings do not cache the resource data, and may be used
for compressible strings. The assumptions about their lengths is
invalid and may lead to oob reads.

R=jkummerow@chromium.org
BUG=v8:4923,chromium:604897
LOG=N

Review URL: https://codereview.chromium.org/1901573003

Cr-Commit-Position: refs/heads/master@{#35660}