ao/pulse: flush stream on underrun

The underflow callback introduced in d27ad96 can be called
when the buffer is still full, causing playback to never
resume afterwards since get_state() reports free_samples == 0.
Fix this by fully resetting on underrun, which flushes
the stream and ensures free buffer space.

fixes #7874

audio/out/ao_pulse.c

@@ -635,7 +635,7 @@ static void audio_get_state(struct ao *ao, struct mp_pcm_state *state)
     // Otherwise, PA will keep hammering us for underruns (which it does instead
     // of stopping the stream automatically).
     if (!state->playing)
-        cork(ao, true);
+        reset(ao);
 }
 
 /* A callback function that is called when the