Fix GCC 4.7 warnings.
Fix GCC 4.7 warnings.

* src/string-search.h (StringSearch): Avoid -Werror=strict-overflow
  warning.

Review URL: http://codereview.chromium.org/8947022
Patch from Tobias Burnus <burnus@net-b.de>.

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@10295 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh