fixing wmv1 slice decoding

Originally committed as revision 1031 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/msmpeg4.c

@@ -693,7 +693,7 @@ static inline int msmpeg4_pred_dc(MpegEncContext * s, int n,
     b = dc_val[ - 1 - wrap];
     c = dc_val[ - wrap];
     
-    if(s->first_slice_line && (n&2)==0){
+    if(s->first_slice_line && (n&2)==0 && s->msmpeg4_version!=4){
         b=c=1024;
     }