pvcalls-front: don't try to free unallocated rings

inflight_req_id is 0 when initialized. If inflight_req_id is 0, there is
no accept_map to free. Fix the check in pvcalls_front_release.
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/pvcalls-front.c

@@ -1030,8 +1030,8 @@ int pvcalls_front_release(struct socket *sock)
 		spin_lock(&bedata->socket_lock);
 		list_del(&map->list);
 		spin_unlock(&bedata->socket_lock);
-		if (READ_ONCE(map->passive.inflight_req_id) !=
-		    PVCALLS_INVALID_ID) {
+		if (READ_ONCE(map->passive.inflight_req_id) != PVCALLS_INVALID_ID &&
+			READ_ONCE(map->passive.inflight_req_id) != 0) {
 			pvcalls_front_free_map(bedata,
 					       map->passive.accept_map);
 		}