Security fix: integer overflow on checking image size

Test is left in another CL (codereview.chromiu,.org/11274036) to avoid conflict there.  Hope it's fine.

BUG=160926


Review URL: https://chromiumcodereview.appspot.com/11410081

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167882 0039d316-1c4b-4281-b951-d872f2087c98
bool ImageDataNaClBackend::IsMapped() const {
  return map_count_ > 0;
}

bool ImageDataNaClBackend::IsMapped() const {
  return map_count_ > 0;
}
