Short answer: NO.
Did a quick google search on the subject and couldn't find a shred of evidence suggesting that a cat is in danger from a bird of prey. In fact, one website went so far as to say: "A Bird of Prey is not stupid! Unless a wild cat was sick and dying a Bird of Prey will avoid this natural predator, as cats are responsible for 90%+ of all wild bird kills."
But to be sure, you could always gun down the buzzard with an air rifle...