Is it safe to do teeth whitening if some of my teeth fillings currently have decay?

First the foundation, then the roof !
Please have all your cavities fixed and then enjoy a laser whitening after you are free of cavities. You do not want the liquid to get into the cavities, especially when they are bigger and open cavities,
Joseph Goodman DDS, Beverly Hills

It is always a better idea to fill the teeth first. Teeth whitening solution going into a cavity could be extremely uncomfortable, no matter how careful the dentist is, there is a chance of this happening.

It usually is not a good idea to start whitening before taking care of cavities, mostly because the cavities can get very sensitive with whitening materials.
But in case of bonding/ filling on the front teeth , sometimes we recommend to whiten the teeth & then match the bondings/fillings to the new shade.