Avoid an overflow in harfbuzz-myanmar.c

Adds a guard to prevent invalid assignment.

BUG=108006

TEST=manually with ASAN.


Review URL: http://codereview.chromium.org/8997001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@115859 0039d316-1c4b-4281-b951-d872f2087c98