audio: fix segfault when yanking USB DAC

The ao_c pointer was stale after the mpctx entry was freed / NULLed.
This prevented the correct early exit from fill_audio_out_buffers.

player/audio.c

@@ -884,6 +884,7 @@ void fill_audio_out_buffers(struct MPContext *mpctx)
                 }
             }
             reinit_audio_filters_and_output(mpctx);
+            ao_c = mpctx->ao_chain;
         }
     }