avfilter/avcodec: use NULL instead of 0 for pointer

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avcodec.c

@@ -53,7 +53,7 @@ AVFilterBufferRef *avfilter_get_audio_buffer_ref_from_frame(const AVFrame *frame
     int64_t layout = av_frame_get_channel_layout(frame);
 
     if (layout && av_get_channel_layout_nb_channels(layout) != av_frame_get_channels(frame)) {
-        av_log(0, AV_LOG_ERROR, "Layout indicates a different number of channels than actually present\n");
+        av_log(NULL, AV_LOG_ERROR, "Layout indicates a different number of channels than actually present\n");
         return NULL;
     }