They would be liable if they were at fault in putting a foreign object--the fig stem--into the pizza, including doing so carelessly. So if this was a pizza that had figs, they would not be liable, since there is nothing wrong with having a fig stem in a pizza with figs. If you or someone sitting with you had ordered or was eating something with figs and got the stem on the pizza (so the restaurant did not do it), they would not be liable. But if they were the reason that the unexpected foreign object was in the pizza, then they would most likely be liable for the damage it did.