aout: reset volume to 1 on module unload

This fixes a corner case where the old volume of an output plugin using
software amplification would be wrongly applied to the next output
plugin instance (if that used "hardware" amplification).

src/audio_output/output.c

@@ -213,6 +213,7 @@ void aout_OutputDelete (audio_output_t *aout)
     aout->pf_flush = NULL;
     aout->pf_volume_set = NULL;
     owner->module = NULL;
+    owner->volume.multiplier = 1.0;
     aout_FiltersDestroyPipeline (owner->filters, owner->nb_filters);
 }