Fixed issue found by clusterfuzz

An integer overflow was causing an issue when reading a string with a very large (or negative) size.

BUG=367764
R=senorblanco@google.com, senorblanco@chromium.org, reed@google.com, borenet@google.com

Author: sugoi@chromium.org

Review URL: https://codereview.chromium.org/255693003

git-svn-id: http://skia.googlecode.com/svn/trunk@14434 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed