avcodec/mpegvideo_enc: add logging context to log

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

libavcodec/mpegvideo_enc.c

@@ -165,7 +165,7 @@ void ff_convert_matrix(MpegEncContext *s, int (*qmat)[64],
         }
     }
     if (shift) {
-        av_log(NULL, AV_LOG_INFO,
+        av_log(s->avctx, AV_LOG_INFO,
                "Warning, QMAT_SHIFT is larger than %d, overflows possible\n",
                QMAT_SHIFT - shift);
     }