Bug 803385. Make sure cookie value is not undefined before trying to trim it. r=felipe a=gavin

toolkit/components/social/WorkerAPI.jsm

@@ -69,7 +69,7 @@ WorkerAPI.prototype = {
       cookies.forEach(function(aCookie) {
         let [name, value] = aCookie.split("=");
         results.push({name: unescape(name.trim()),
-                      value: unescape(value.trim())});
+                      value: value ? unescape(value.trim()) : ""});
       });
       this._port.postMessage({topic: "social.cookies-get-response",
                               data: results});