correct preview mode behaviour of fivestar.module

Fivestar module seems to have a small but annoying bug:
when previewing a node enabled for fivestar rating, fivestar.module assumes a node-id on line 390, which doesn't exist in this case, and triggers node_load with an empty string instead of node-id or id array.
Thus, a mysql error occurs and further php warnings are thrown.

the attached patch should be a simple fix for this bug, simply outputing no fivestar form when in preview mode.