lavfi: add missing check in avfilter_filter_samples()

Avoid out-of-buffer data access when nb_channels is 8.

libavfilter/avfilter.c

@@ -678,7 +678,7 @@ void avfilter_filter_samples(AVFilterLink *link, AVFilterBufferRef *samplesref)
         link->cur_buf->audio->sample_rate = samplesref->audio->sample_rate;
 
         /* Copy actual data into new samples buffer */
-        for (i = 0; samplesref->data[i]; i++)
+        for (i = 0; samplesref->data[i] && i < 8; i++)
             memcpy(link->cur_buf->data[i], samplesref->data[i], samplesref->linesize[0]);
 
         avfilter_unref_buffer(samplesref);