examples/encode_video: only add sequence end code for mpeg1/2 video

Only add sequence end code for mpeg1/mpeg2 video, or else use the encoder
libx264 or libx265 in this sample, decoding the output file will get
unknow NALU type error.

Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

doc/examples/encode_video.c

@@ -186,7 +186,8 @@ int main(int argc, char **argv)
     encode(c, NULL, pkt, f);
 
     /* add sequence end code to have a real MPEG file */
-    fwrite(endcode, 1, sizeof(endcode), f);
+    if (codec->id == AV_CODEC_ID_MPEG1VIDEO || codec->id == AV_CODEC_ID_MPEG2VIDEO)
+        fwrite(endcode, 1, sizeof(endcode), f);
     fclose(f);
 
     avcodec_free_context(&c);