Fix invalid use of the PulseAudio stream from the context callback

Make sure the context callback is registered only after the stream
is created, and unregistered before the stream is destroyed. This
should fix a potential assertion failure when stopping audio.

modules/audio_output/pulse.c

@@ -842,15 +842,6 @@ static int Open(vlc_object_t *obj)
     sys->desync = 0;
     sys->rate = ss.rate;
 
-    /* Context events */
-    const pa_subscription_mask_t mask = PA_SUBSCRIPTION_MASK_SINK
-                                      | PA_SUBSCRIPTION_MASK_SINK_INPUT;
-
-    pa_context_set_subscribe_callback(ctx, context_cb, aout);
-    op = pa_context_subscribe(ctx, mask, NULL, NULL);
-    if (likely(op != NULL))
-       pa_operation_unref(op);
-
     /* Channel volume */
     sys->base_volume = PA_VOLUME_NORM;
     pa_cvolume_set(&sys->cvolume, ss.channels, PA_VOLUME_NORM);
@@ -940,6 +931,14 @@ static int Open(vlc_object_t *obj)
     if (op != NULL)
         pa_operation_unref(op);
     stream_moved_cb(s, aout);
+
+    /* Context events */
+    const pa_subscription_mask_t mask = PA_SUBSCRIPTION_MASK_SINK
+                                      | PA_SUBSCRIPTION_MASK_SINK_INPUT;
+    pa_context_set_subscribe_callback(ctx, context_cb, aout);
+    op = pa_context_subscribe(ctx, mask, NULL, NULL);
+    if (likely(op != NULL))
+       pa_operation_unref(op);
     pa_threaded_mainloop_unlock(sys->mainloop);
 
     aout->format.i_format = format;
@@ -984,6 +983,7 @@ static void Close (vlc_object_t *obj)
         pa_stream_set_started_callback(s, NULL, NULL);
         pa_stream_set_suspended_callback(s, NULL, NULL);
         pa_stream_set_underflow_callback(s, NULL, NULL);
+        pa_context_set_subscribe_callback(ctx, NULL, NULL);
 
         pa_stream_unref(s);
         pa_threaded_mainloop_unlock(sys->mainloop);