Fix OSX crash in JAWTWindow.invalidateNative

In a fairly large program, it appears that invalidateNative() is called
after unlockSurfaceImpl() has been called. The latter does
'ds.FreeDrawingSurfaceInfo(dsi)' which supposedly triggers OS X to
collect the object to which jawtSurfaceLayersHandle was pointing. Then
invalidateNative() will send a message to a non-existing
'surfaceLayers' instance.