dccp_recv: Avoid NULL pointer dereference.

dccp_recv.c:48:16: warning: Access to field 'btrp' results in a dereference of a null pointer (loaded from variable 'pdd')
        btr_pool_free(pdd->btrp);
                      ^~~

In dccp_recv_close(), if pdd is NULL, we avoid closing pdd->fd but
dereference pdd nevertheless one line later. Fix this by returning
early if pdd is NULL.