avfilter/avf_showwaves: fix off by one error in loop condition

It caused segfaults.
Signed-off-by: Marton Balint <cus@passwd.hu>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avf_showwaves.c

@@ -157,7 +157,7 @@ inline static int push_frame(AVFilterLink *outlink)
         showwaves->req_fullfilled = 1;
     showwaves->outpicref = NULL;
     showwaves->buf_idx = 0;
-    for (i = 0; i <= nb_channels; i++)
+    for (i = 0; i < nb_channels; i++)
         showwaves->buf_idy[i] = 0;
     return ret;
 }