sso: free signed_data on error

found when playing with https://github.com/facebook/infer

src/sso/sso.c

@@ -281,6 +281,7 @@ int sso_ticket_sign(sso_ticket_t t, const unsigned char *secret_key, char *out,
                   (unsigned long long)serialized_size,
                   secret_key) != 0) {
     free(serialized);
+    free(signed_data);
     return SSO_ERR_SIGNATURE;
   }