avfilter/af_headphone: Fix stack buffer overflow

The number of channels can be up to 64, not only 16.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit 58b6594b01)

libavfilter/af_headphone.c

@@ -187,7 +187,7 @@ static int headphone_convolute(AVFilterContext *ctx, void *arg, int jobnr, int n
     const int in_channels = in->channels;
     const int buffer_length = s->buffer_length;
     const uint32_t modulo = (uint32_t)buffer_length - 1;
-    float *buffer[16];
+    float *buffer[64];
     int wr = *write;
     int read;
     int i, l;