flvdec: check return value of create_stream()

Fixes null pointer dereference. Fixes CID732242.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavformat/flvdec.c

@@ -730,6 +730,8 @@ static int flv_read_packet(AVFormatContext *s, AVPacket *pkt)
         av_log(s, AV_LOG_WARNING, "Stream discovered after head already parsed\n");
         st = create_stream(s,
              (int[]){AVMEDIA_TYPE_VIDEO, AVMEDIA_TYPE_AUDIO, AVMEDIA_TYPE_DATA}[stream_type]);
+        if (!st)
+            return AVERROR(ENOMEM);
 
     }
     av_dlog(s, "%d %X %d \n", stream_type, flags, st->discard);