4xm decoder: fix data size for i2 frames.

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

libavcodec/4xm.c

@@ -829,7 +829,7 @@ static int decode_frame(AVCodecContext *avctx,
 
     if(frame_4cc == AV_RL32("ifr2")){
         p->pict_type= AV_PICTURE_TYPE_I;
-        if(decode_i2_frame(f, buf-4, frame_size) < 0){
+        if(decode_i2_frame(f, buf-4, frame_size+4) < 0){
             av_log(f->avctx, AV_LOG_ERROR, "decode i2 frame failed\n");
             return -1;
         }