ffplay: silence buffer size must be a multiple of frame size

Otherwise the audio channels sometimes would be shifted on output.
Signed-off-by: Marton Balint <cus@passwd.hu>

ffplay.c

@@ -2163,6 +2163,7 @@ static void sdl_audio_callback(void *opaque, Uint8 *stream, int len)
     VideoState *is = opaque;
     int audio_size, len1;
     int bytes_per_sec;
+    int frame_size = av_samples_get_buffer_size(NULL, is->audio_tgt_channels, 1, is->audio_tgt_fmt, 1);
     double pts;
 
     audio_callback_time = av_gettime();
@@ -2173,7 +2174,7 @@ static void sdl_audio_callback(void *opaque, Uint8 *stream, int len)
            if (audio_size < 0) {
                 /* if error, just output silence */
                is->audio_buf      = is->silence_buf;
-               is->audio_buf_size = sizeof(is->silence_buf);
+               is->audio_buf_size = sizeof(is->silence_buf) / frame_size * frame_size;
            } else {
                if (is->show_mode != SHOW_MODE_VIDEO)
                    update_sample_display(is, (int16_t *)is->audio_buf, audio_size);