yop: check return value of avformat_new_stream()

Fixes null pointer dereference, fixes CID703729.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavformat/yop.c

@@ -64,6 +64,8 @@ static int yop_read_header(AVFormatContext *s)
 
     audio_stream = avformat_new_stream(s, NULL);
     video_stream = avformat_new_stream(s, NULL);
+    if (!audio_stream || !video_stream)
+        return AVERROR(ENOMEM);
 
     // Extra data that will be passed to the decoder
     video_stream->codec->extradata_size = 8;