No need to test for p_sys != NULL as it's already tested in the constructor.

modules/audio_filter/channel_mixer/mono.c

@@ -558,8 +558,6 @@ static void stereo2mono_downmix( aout_filter_t * p_filter,
     p_out = p_out_buf->p_buffer;
     i_out_size = p_out_buf->i_nb_bytes;
 
-    if( p_sys != NULL )
-    {
     /* Slide the overflow buffer */
     p_overflow = p_sys->p_overflow_buffer;
     i_overflow_size = p_sys->i_overflow_buffer_size;
@@ -631,11 +629,6 @@ static void stereo2mono_downmix( aout_filter_t * p_filter,
             }
         }
     }
-    }
-    else
-    {
-        memset( p_out, 0, i_out_size );
-    }
 }
 
 /* Simple stereo to mono mixing. */