Don't crash if passed a NULL overlay.  The app crashes anyway, since
it's not checking the return value of the create call, but at least it's
not crashing in SDL anymore. :)

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402543