spatializer: prepare format on success

This fixes a i_bytes_per_frame/i_frame_length inconsistency when chaining more
than one audio filters.

modules/audio_filter/spatializer/spatializer.cpp

@@ -160,6 +160,7 @@ static int Open( vlc_object_t *p_this )
     }
 
     p_filter->fmt_in.audio.i_format = VLC_CODEC_FL32;
+    aout_FormatPrepare(&p_filter->fmt_in.audio);
     p_filter->fmt_out.audio = p_filter->fmt_in.audio;
     p_filter->pf_audio_filter = DoWork;
     return VLC_SUCCESS;