Fix out-of-bounds read in new transform parser

I made a subtle mistake in the new fast-path CSS transform parser and
accidentially read two bytes out of bounds. This CL fixes the bounds check.

BUG=331254

Review URL: https://codereview.chromium.org/118723007

git-svn-id: svn://svn.chromium.org/blink/trunk@164559 bbb929c8-8fbe-4397-9dbb-9b2b20218538