avcodec/mpegvideo_enc: Add check for av_packet_new_side_data()

Add check for av_packet_new_side_data() to avoid null pointer
dereference if allocation fails.

Fixes: bdc1220eeb ("h263enc: Add an option for outputting info about MBs as side data")
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@outlook.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/mpegvideo_enc.c

@@ -1825,6 +1825,8 @@ int ff_mpv_encode_picture(AVCodecContext *avctx, AVPacket *pkt,
             s->mb_info_ptr = av_packet_new_side_data(pkt,
                                  AV_PKT_DATA_H263_MB_INFO,
                                  s->mb_width*s->mb_height*12);
+            if (!s->mb_info_ptr)
+                return AVERROR(ENOMEM);
             s->prev_mb_info = s->last_mb_info = s->mb_info_size = 0;
         }