avcodec/pngdec: add logging context to log

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

libavcodec/pngdec.c

@@ -424,7 +424,7 @@ static int png_decode_idat(PNGDecContext *s, int length)
             s->zstream.next_out  = s->crow_buf;
         }
         if (ret == Z_STREAM_END && s->zstream.avail_in > 0) {
-            av_log(NULL, AV_LOG_WARNING,
+            av_log(s->avctx, AV_LOG_WARNING,
                    "%d undecompressed bytes left in buffer\n", s->zstream.avail_in);
             return 0;
         }