player: allow changing playback speed in early audio init stages

If the audio decoder was created, but no audio filter chain created yet
(still trying to decode a first audio frame), setting the "speed"
property could explode. It tried to recreate the filter chain, even
though no format was set yet.

This is inconvenient and should not happen.

player/audio.c

@@ -135,7 +135,7 @@ void set_playback_speed(struct MPContext *mpctx, double new_speed)
 
     opts->playback_speed = new_speed;
 
-    if (!mpctx->d_audio)
+    if (!mpctx->d_audio || mpctx->d_audio->afilter->initialized < 1)
         return;
 
     recreate_audio_filters(mpctx);