do not fail on -x loopback if no audio

There is no audio loopback protocol. But when audio is dot enabled,
we can silently ignore that and keep running only video. Fixes:
`uv -t testcard -d gl -x loopback`

(the above, however, worked when used `-x V:loopback`)

src/audio/audio.cpp

@@ -451,7 +451,7 @@ int audio_init(struct state_audio **ret, struct module *parent,
                 s->sender = NET_JACK;
                 s->receiver = NET_JACK;
 #endif
-        } else {
+        } else if (s->audio_tx_mode != 0) {
                 log_msg(LOG_LEVEL_ERROR, "Unknow audio protocol: %s\n", opt->proto);
                 goto error;
         }