avformat/a64: Avoid segfault in a64_write_header() when stream codec is not open

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/a64.c

@@ -40,7 +40,7 @@ static int a64_write_header(AVFormatContext *s)
         return AVERROR_INVALIDDATA;
     }
 
-    switch (avctx->codec->id) {
+    switch (avctx->codec_id) {
     case AV_CODEC_ID_A64_MULTI:
         header[2] = 0x00;
         header[3] = AV_RB32(avctx->extradata+0);