carbon: fix crash in gradient draw

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42850 4005530d-fff6-0310-9dd1-cebe43e6787f
carbon: fix linear gradient

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@42851 4005530d-fff6-0310-9dd1-cebe43e6787f