sd-rtnl: don't treat 0-length messages specially

Still parse the CMSG data, and most importantly make sure we drop the message when peeking.

src/libsystemd/sd-rtnl/rtnl-message.c

@@ -1350,10 +1350,6 @@ static int socket_recv_message(int fd, struct iovec *iov, uint32_t *_group, bool
                         log_debug("rtnl: no data in socket");
 
                 return (errno == EAGAIN || errno == EINTR) ? 0 : -errno;
-        } else if (r == 0) {
-                /* connection was closed by the kernel? */
-                log_warning("rtnl: ignoring empty message");
-                return 0;
         }
 
         for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {