avfilter/af_headphone: Remove pointless additions

buffer_length is a power-of-two and modulo is buffer_length - 1, so that
buffer_length & modulo is zero.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavfilter/af_headphone.c

@@ -178,7 +178,7 @@ static int headphone_convolute(AVFilterContext *ctx, void *arg, int jobnr, int n
                 continue;
             }
 
-            read = (wr - (ir_len - 1) + buffer_length) & modulo;
+            read = (wr - (ir_len - 1)) & modulo;
 
             if (read + ir_len < buffer_length) {
                 memcpy(temp_src, bptr + read, ir_len * sizeof(*temp_src));