4xm: return error code if decode_init() failed

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/4xm.c

@@ -941,7 +941,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
 
     if (avctx->extradata_size != 4 || !avctx->extradata) {
         av_log(avctx, AV_LOG_ERROR, "extradata wrong or missing\n");
-        return 1;
+        return AVERROR_INVALIDDATA;
     }
     if((avctx->width % 16) || (avctx->height % 16)) {
         av_log(avctx, AV_LOG_ERROR, "unsupported width/height\n");