[Harfbuzz] Fix OOB read in tibetan_form()

      |c| should be smaller than 0x0fc0.

BUG=chromium:122586
TEST=manual


Review URL: http://codereview.chromium.org/10024052

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@131694 0039d316-1c4b-4281-b951-d872f2087c98