audiofilters: Do not try to insert user audio-filters when our target is...

audiofilters: Do not try to insert user audio-filters when our target is SPDIF. We currently have no way to apply any of our filters to the encoded signal.

Cherry-picked from [e5587029]

src/audio_output/input.c

@@ -218,7 +218,7 @@ int aout_InputNew( aout_instance_t * p_aout, aout_input_t * p_input )
     psz_visual = val.psz_string;
 
     /* parse user filter lists */
-    for( i_visual = 0; i_visual < 2; i_visual++ )
+    for( i_visual = 0; i_visual < 2 && !AOUT_FMT_NON_LINEAR(&chain_output_format); i_visual++ )
     {
         char *psz_next = NULL;
         char *psz_parser = i_visual ? psz_visual : psz_filters;