Yes, its safety when used as a food additive has been approved by the U.S. Food and Drug Administration (FDA), European Food Safety Authority (EFSA),  Joint FAO/WHO Expert Committee on Food Additives (JECFA), as well as other authorities.

FDA
It is GENERALLY RECOGNIZED AS SAFE (GRAS) and can be used as an antimicrobial agent, flavoring agent and adjuvant, and pH control agent in food at levels not to exceed current good manufacturing practice.

The following are the approved food list and with the max usage (6):

Baked goods: 0.4%
Gravies and sauces: 0.25%
Fats and oils: 0.1%
Meat products: 0.1%
Soft candy: 0.1%
Snack foods, soups and soup mixes: 0.05%
EFSA
Sodium diacetate (E262(ii)) is authorised as a food additive in the European Union (EU) in accordance with Annex II and Annex III to Regulation (EC) No 1333/2008 on food additives and categorized as “additives other than colours and sweeteners” (7)

Uses
Its approved applications are listed separately with E262(ii) or in Group I where the uses is quantum satis (QS).

Following foods may contain it (8):

Canned or bottled fruit and vegetables
Bread
Pain courant français; Friss búzakenyér, fehér és félbarna kenyerek
Processed cereal-based foods and baby foods (a pH adjustment)

http://www.chinafooding.com/Sodium_Diacetate