Don't call loadAAT() if createFont() returned NULL

If createFont() returned NULL this could mean that FreeType failed to
load the font, though Core Text might still be able to load the font, it
will fail later in xdvipdfmx which must open the font with FreeType, so
we better fail early.
Fix letterspacing with AAT fonts

Now using similar code to the OT case, no to find a away to de-duplicate
it, though.
Simplify

Use CTRunGetTypographicBounds() to get run width.
Minor
This check is not needed with Core Text