WMADEC: fix ticket125

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

libavcodec/wmadec.c

@@ -103,7 +103,7 @@ static int wma_decode_init(AVCodecContext * avctx)
     s->use_variable_block_len = flags2 & 0x0004;
 
     if(avctx->codec->id == CODEC_ID_WMAV2 && avctx->extradata_size >= 8){
-        if(!AV_RL16(extradata+6) && s->use_variable_block_len){
+        if(AV_RL16(extradata+4)==0xd && s->use_variable_block_len){
             av_log(avctx, AV_LOG_WARNING, "Disabling use_variable_block_len, if this fails contact the ffmpeg developers and send us the file\n");
             s->use_variable_block_len= 0; // this fixes issue1503
         }