Fix a heap buffer overaflow in ucstrTextExtract

Merge the upstream patch http://bugs.icu-project.org/trac/changeset/29356

BUG=146892
TEST=See the bug report.
Review URL: https://codereview.chromium.org/10957050

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/icu46@158118 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
3 files changed